Annual report pursuant to Section 13 and 15(d)

Accumulated Other Comprehensive Income (Loss) - Activity in Accumulated Other Comprehensive Income (Detail)

v3.8.0.1
Accumulated Other Comprehensive Income (Loss) - Activity in Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period, before tax $ (4,089) $ (2,331)  
Balance at end of period, before tax (3,188) (4,089) $ (2,331)
Beginning balance 531,025 493,918 453,387
Other comprehensive loss before reclassification 2,273 (2,033) 5,833
Reclassification from accumulated other comprehensive loss (1,748) (46) (6,611)
Other comprehensive income (loss) 525 (2,079) (778)
Ending balance 562,477 531,025 493,918
Accumulated Net Unrealized Investment Gain (Loss) [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period, before tax (4,089) (2,331) (985)
Other comprehensive loss before reclassification, before tax 2,649 (1,712) 5,265
Reclassification from accumulated other comprehensive loss, before tax (1,748) (46) (6,611)
Period change, before tax 901 (1,758) (1,346)
Balance at end of period, before tax   (4,089) (2,331)
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Interest-Only Strip [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period, before tax 0 9 16
Other comprehensive loss before reclassification, before tax 0 (9) (7)
Reclassification from accumulated other comprehensive loss, before tax 0 0
Period change, before tax 0 (9) (7)
Balance at end of period, before tax 0 0 9
Tax (Expense) Benefit [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period, tax 1,695 2,007 1,432
Other comprehensive loss before reclassification, tax (376) (312) 575
Reclassification from accumulated other comprehensive loss, tax 0 0 0
Period change, tax (376) (312) 575
Balance at end of period, tax 1,319 1,695 2,007
Accumulated Other Comprehensive Income (Loss) [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance (2,394) (315) 463
Other comprehensive income (loss) 525 (2,079) (778)
Ending balance $ (1,869) $ (2,394) $ (315)