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
Statement of Comprehensive Income [Abstract]
Net income	$ 52,579	$ 51,536	$ 52,503	$ 49,722	$ 51,006	$ 50,457	$ 47,834	$ 50,429	$ 206,340	$ 199,726	$ 179,456
Other comprehensive (loss) income, net of tax:
Total available-for-sale and transferred securities									(22,828)	19,038	(45,358)
Total derivative instruments									2,550	(7,324)	9,696
Total defined benefit pension and postretirement benefit plans									(1,567)	(19,426)	19,379
Other comprehensive loss, net of tax									(21,845)	(7,712)	(16,283)
Comprehensive income									$ 184,495	$ 192,014	$ 163,173