CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 13,474	$ 10,995	$ 34,934	$ 32,553
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale		(2,641)	3,729	10,612	3,582
Related income tax (expense) benefit		1,036	(1,472)	(4,227)	(1,414)
Net investment gains reclassified into earnings		0	(1)	(1,919)	(20)
Tax expense		0	0	765	8
Net effect on other comprehensive income (loss)		(1,605)	2,256	5,231	2,156
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	295	240	870	791
Related income tax (expense) benefit		(111)	(103)	(339)	(323)
Net effect on other comprehensive income for the period		184	137	531	468
Total other comprehensive income (loss)		(1,421)	2,393	5,762	2,624
Comprehensive income		$ 12,053	$ 13,388	$ 40,696	$ 35,177

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