Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 69,893	$ 64,496	$ 61,579	$ 59,471	$ 57,660	$ 51,817	$ 50,603	$ 47,047	$ 255,439	$ 207,127	$ 204,729
Other comprehensive (loss) income, net of tax:
Total available-for-sale and transferred securities									(7,590)	(9,069)	(22,828)
Total derivative instruments									4,565	5,912	2,550
Total defined benefit pension and postretirement benefit plans									4,135	4,270	(1,567)
Other comprehensive income (loss), net of tax									1,110	1,113	(21,845)
Comprehensive income									$ 256,549	$ 208,240	$ 182,884