CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income	$ 21,292	$ 4,129	$ 6,128	$ 14,654	$ 9,895	$ 11,189	$ 8,748	$ 20,330	$ 46,203	$ 50,162	$ 29,992
OTHER COMPREHENSIVE INCOME (LOSS)
Unrealized holding gains (losses) on investment securities, net of deferred income tax expense (benefit) of $(2,092), $(7,021) and $18,664									(3,885)	(13,037)	34,663
Reclassification adjustment for realized investment gains included in net income, net of income tax expense of $(3,628), $(2,668) and $(2,518)									(6,736)	(4,956)	(4,677)
Reclassification adjustment for amounts amortized into net periodic pension and postretirement benefit income, net of deferred income tax expense of $(457), $(693) and $(955):
Net actuarial loss									1,549	863	375
Prior service credit									(2,399)	(2,150)	(2,149)
Total reclassification adjustment associated with affiliate's pension and postretirement benefit plans									(850)	(1,287)	(1,774)
Change in funded status of affiliate's pension and postretirement benefit plans, net of deferred income tax benefit of $(474), $(2,126) and $(2,994):
Net actuarial loss									(542)	(3,637)	(5,525)
Prior service cost									(339)	(312)	(35)
Total change in funded status of affiliate's pension and postretirement benefit plans									(881)	(3,949)	(5,560)
Other comprehensive income (loss)									(12,352)	(23,229)	22,652
Total comprehensive income									$ 33,851	$ 26,933	$ 52,644