CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income		$ 45,355	$ 38,200	$ 44,422
Investments available-for-sale:
Net change in unrealized gains (losses) on investments available-for-sale		(2,556)	12,807	(33,214)
Related income tax (expense) benefit		1,022	(5,090)	13,245
Net investment gains reclassified into earnings		36	5	115
Related income tax expense		(14)	(2)	(46)
Net effect on other comprehensive income (loss)		(1,512)	7,720	(19,900)
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	1,736	(9,235)	9,340
Related income tax (expense) benefit		(698)	3,662	(3,722)
Net effect on other comprehensive income (loss)		1,038	(5,573)	5,618
Total other comprehensive income (loss)		(474)	2,147	(14,282)
Comprehensive income		$ 44,881	$ 40,347	$ 30,140

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