CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income		$ 48,250	$ 45,355	$ 38,200
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale		(6,246)	(2,520)	12,812
Related income tax (expense) benefit		2,484	1,030	(5,089)
Net investment gains reclassified into earnings		(1,932)	(36)	(5)
Tax expense		770	14	2
Net effect on other comprehensive income (loss)		(4,924)	(1,512)	7,720
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	(651)	1,736	(9,235)
Related income tax (expense) benefit		258	(698)	3,662
Net effect on other comprehensive income for the period		(393)	1,038	(5,573)
Total other comprehensive income (loss)		(5,317)	(474)	2,147
Comprehensive income		$ 42,933	$ 44,881	$ 40,347

[1] This amount is included in the computation of net periodic benefit cost, see Note 13