CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 9,895	$ 11,189	$ 8,748	$ 20,330	$ 16,154	$ 2,229	$ 1,014	$ 10,595	$ 50,162	$ 29,992	$ 43,519
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income tax expense (benefit) of $(7,021), $18,664 and $(8,390)									(13,037)	34,663	(15,582)
Reclassification adjustment for realized investment gains included in net income, net of income tax expense of $(2,668), $(2,518) and $(3,149)									(4,956)	(4,677)	(5,848)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit cost (income), net of deferred income tax (expense) benefit of $(693), $(955) and $765:
Net actuarial loss									863	375	1,882
Prior service credit									(2,150)	(2,149)	(460)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(1,287)	(1,774)	1,422
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income tax expense (benefit) of $(2,126), $(2,994) and $16,836:
Net actuarial gain (loss)									(3,637)	(5,525)	13,718
Prior service (cost) credit									(312)	(35)	17,548
Total change in funded status of affiliate's pension and postretirement benefit plans									(3,949)	(5,560)	31,266
Other comprehensive income (loss)									(23,229)	22,652	11,258
Total comprehensive income									$ 26,933	$ 52,644	$ 54,777