Accumulated Other Comprehensive Loss - Changes (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pre-tax Amount
Other comprehensive income (loss)	$ 64,007,000	$ (24,263,000)	$ 137,143,000	$ (90,225,000)
Income Tax Benefit (Expense)
Other comprehensive income (loss)	(17,237,000)	6,537,000	(36,932,000)	24,266,000
Net of tax
Balance, beginning	2,613,202,000	2,520,862,000	2,524,839,000	2,532,551,000
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
Early Adoption | ASU 2018-02
Income Tax Benefit (Expense)
Early adoption of ASU No. 2018-02				(20,068,000)
Net of tax
Other comprehensive income (loss)				(20,068,000)
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Balance	(107,779,000)	(225,385,000)	(180,915,000)	(159,423,000)
Balance	(43,772,000)	(249,648,000)	(43,772,000)	(249,648,000)
Income Tax Benefit (Expense)
Balance	29,025,000	60,701,000	48,720,000	63,040,000
Balance	11,788,000	67,238,000	11,788,000	67,238,000
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) | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)				(20,068,000)
Pension and other benefits, net actuarial losses arising during the year
Pre-tax Amount
Other comprehensive income (loss)	(813,000)		(813,000)
Income Tax Benefit (Expense)
Other comprehensive income (loss)	219,000		219,000
Net of tax
Other comprehensive income (loss)	(594,000)		(594,000)
Pension and other benefits, net change in pension and other benefits
Pre-tax Amount
Other comprehensive income (loss)	(813,000)		(813,000)
Income Tax Benefit (Expense)
Other comprehensive income (loss)	219,000		219,000
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 | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)				(5,393,000)
Investment securities:
Pre-tax Amount
Unrealized net (losses) gains arising during the period	64,841,000	(24,688,000)	135,364,000	(91,388,000)
Reclassification of net losses (gains) to net income	(21,000)		2,592,000
Other comprehensive income (loss)	(64,820,000)	(24,688,000)	137,956,000	(91,388,000)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(17,462,000)	6,649,000	(36,453,000)	24,572,000
Reclassification of net losses (gains) to net income	6,000		(698,000)
Other comprehensive income (loss)	(17,456,000)	6,649,000	(37,151,000)	24,572,000
Net of tax
Balance, beginning	(50,375,000)	(138,334,000)	(103,816,000)	(74,117,000)
Unrealized net gains (losses) arising during the period	47,379,000	(18,039,000)	98,911,000	(66,816,000)
Reclassification of net losses (gains) to net income	(15,000)		1,894,000
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: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)				(15,440,000)
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net (losses) gains arising during the period		425,000		1,163,000
Other comprehensive income (loss)		425,000		1,163,000
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period		(112,000)		(306,000)
Other comprehensive income (loss)		(112,000)		(306,000)
Net of tax
Balance, beginning		4,989,000		3,680,000
Unrealized net gains (losses) arising during the period		313,000		857,000
Other comprehensive income (loss)		313,000		857,000
Balance, ending		$ 5,302,000		5,302,000
Cash flow derivative hedges: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)				$ 765,000