CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 38,200,000		$ 44,422,000		$ 36,554,000
Investments available-for-sale:
Net change in unrealized gains (losses) on investments available-for-sale	12,807,000		(33,214,000)		(40,000)
Related income tax (expense) benefit	(5,090,000)		13,245,000		16,000
Net investment gains reclassified into earnings	5,000		115,000		459,000
Related income tax expense	(2,000)		(46,000)		(183,000)
Net effect on other comprehensive income (loss)	7,720,000		(19,900,000)		252,000
Defined benefit pension plan:
Recognition of unrealized gain (loss)	(9,235,000)	[1]	9,340,000	[1]	(3,639,000)	[1]
Related income tax (expense) benefit	3,662,000		(3,722,000)		1,451,000
Net effect on other comprehensive income (loss)	(5,573,000)		5,618,000		(2,188,000)
Total other comprehensive income (loss)	2,147,000		(14,282,000)		(1,936,000)
Comprehensive income	$ 40,347,000		$ 30,140,000		$ 34,618,000

[1] This amount is included in the computation of net periodic benefit cost, see Note 13