CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income		$ 14,741	$ 10,647	$ 29,853	$ 21,460
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale		4,003	2,598	5,505	13,253
Related income tax expense		(1,593)	(1,031)	(2,191)	(5,263)
Net investment gains reclassified into earnings		(1,273)	(150)	(1,275)	(1,919)
Related income tax expense		508	60	508	765
Net effect on other comprehensive income (loss)		1,645	1,477	2,547	6,836
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	295	291	590	575
Related income tax benefit		(118)	(115)	(235)	(228)
Net effect on other comprehensive income for the period		177	176	355	347
Total other comprehensive income		1,822	1,653	2,902	7,183
Comprehensive income		$ 16,563	$ 12,300	$ 32,755	$ 28,643

[1] This amount is included in the computation of net periodic benefit cost, see Note 9.