CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 53,209	$ 48,250	$ 45,355
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale	(294)	(6,246)	(2,520)
Related income tax expense	108	2,484	1,030
Net investment gains reclassified into earnings	(1,273)	(1,932)	(36)
Related income tax expense	504	770	14
Net effect on other comprehensive income (loss)	(955)	(4,924)	(1,512)
Defined benefit pension plan:
Recognition of unrealized gain (loss)	1,202	(651)	1,736
Related income tax (expense) benefit	(490)	258	(698)
Net effect on other comprehensive income for the period	712	(393)	1,038
Total other comprehensive income	(243)	(5,317)	(474)
Comprehensive income	$ 52,966	$ 42,933	$ 44,881