CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 10,647	$ 10,333	$ 21,460	$ 21,558
Investments available-for-sale:
Net change in unrealized gains on investments available-for-sale		2,598	(4,781)	13,253	(147)
Related income tax (expense) benefit		(1,031)	1,899	(5,263)	58
Net investment gains reclassified into earnings		(150)	(19)	(1,919)	(19)
Related income tax expense		60	8	765	8
Net effect on other comprehensive income (loss) for the period		1,477	(2,893)	6,836	(100)
Defined benefit pension plan:
Recognition of unrealized gain (loss)	[1]	291	259	575	551
Related income tax (expense) benefit		(115)	(104)	(228)	(220)
Net effect on other comprehensive income for the period		176	155	347	331
Total other comprehensive income (loss)		1,653	(2,738)	7,183	231
Comprehensive income		$ 12,300	$ 7,595	$ 28,643	$ 21,789

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