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
Statement of Comprehensive Income [Abstract]
Net income	$ 57,660	$ 51,817	$ 50,603	$ 47,047	$ 51,812	$ 51,370	$ 52,043	$ 49,504	$ 207,127	$ 204,729	$ 199,726
Other comprehensive income (loss), net of tax:
Total available-for-sale and transferred securities									(9,069)	(22,828)	19,038
Total derivative instruments									5,912	2,550	(7,324)
Total defined benefit pension and postretirement benefit plans									4,270	(1,567)	(19,426)
Other comprehensive income (loss), net of tax									1,113	(21,845)	(7,712)
Comprehensive income									$ 208,240	$ 182,884	$ 192,014