CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - UNAUDITED - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income		$ 15,089	$ 13,474	$ 44,942	$ 34,934
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale		113	(2,641)	5,618	10,612
Related income tax expense		(45)	1,036	(2,236)	(4,227)
Net investment gains reclassified into earnings		0	0	(1,275)	(1,919)
Related income tax expense		0	0	508	765
Net effect on other comprehensive income (loss)		68	(1,605)	2,615	5,231
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	296	295
Related income tax benefit		(129)	(111)	(364)	(339)
Net effect on other comprehensive income for the period		167	184	522	531
Total other comprehensive income		235	(1,421)	3,137	5,762
Comprehensive income		$ 15,324	$ 12,053	$ 48,079	$ 40,696

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