CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 24,399	$ 14,741	$ 46,064	$ 29,853
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale	(4,230)	4,003	(16,919)	5,505
Related income tax expense	1,107	(1,593)	4,428	(2,191)
Net investment gains reclassified into earnings	0	(1,273)	(63)	(1,275)
Related income tax expense	0	508	16	508
Net effect on other comprehensive income (loss) for the period	(3,123)	1,645	(12,538)	2,547
Defined benefit pension plan:
Recognition of unrealized gain (loss)	250	295	500	590
Related income tax (expense) benefit	(65)	(118)	(184)	(235)
Net effect on other comprehensive income for the period	185	177	316	355
Total other comprehensive income	(2,938)	1,822	(12,222)	2,902
Comprehensive income	$ 21,461	$ 16,563	$ 33,842	$ 32,755