Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net of tax
Balance, beginning	$ 2,613,202,000	$ 2,520,862,000	$ 2,524,839,000	$ 2,532,551,000
Net income	72,433,000	69,053,000	142,357,000	137,011,000
Other comprehensive income (loss)	46,770,000	(17,726,000)	100,211,000	(65,959,000)
Balance, ending	2,659,441,000	2,459,175,000	2,659,441,000	2,459,175,000
ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(20,068,000)
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	(78,754,000)	(164,684,000)	(132,195,000)	(96,383,000)
Other comprehensive income (loss)	46,770,000	(17,726,000)	100,211,000	(65,959,000)
Balance, ending	(31,984,000)	(182,410,000)	(31,984,000)	(182,410,000)
Accumulated other comprehensive Income (loss) | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(20,068,000)
Pension and other benefits, net change in pension and other benefits
Net of tax
Balance, beginning	(28,379,000)	(31,339,000)	(28,379,000)	(25,946,000)
Other comprehensive income (loss)	(594,000)		(594,000)
Balance, ending	(28,973,000)	(31,339,000)	(28,973,000)	(31,339,000)
Pension and other benefits, net change in pension and other benefits | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(5,393,000)
Investment securities:
Net of tax
Balance, beginning	(50,375,000)	(138,334,000)	(103,816,000)	(74,117,000)
Other comprehensive income (loss)	47,364,000	(18,039,000)	100,805,000	(66,816,000)
Balance, ending	$ (3,011,000)	(156,373,000)	$ (3,011,000)	(156,373,000)
Investment securities: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(15,440,000)
Cash flow derivative hedges:
Net of tax
Balance, beginning		4,989,000		3,680,000
Other comprehensive income (loss)		313,000		857,000
Balance, ending		$ 5,302,000		5,302,000
Cash flow derivative hedges: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				$ 765,000