Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pre-tax Amount
Balance	$ (145,472)	$ (84,722)	$ (85,048)
Other comprehensive income (loss)	(13,951)	(60,750)	326
Balance	(159,423)	(145,472)	(84,722)
Income Tax Benefit (Expense)
Balance	57,461	33,463	33,591
Other comprehensive income (loss)	5,579	23,998	(128)
Balance	63,040	57,461	33,463
Net of tax
Balance	2,476,485	2,736,941	2,675,040
Other comprehensive (loss) income	(8,372)	(36,752)	198
Balance	2,532,551	2,476,485	2,736,941
Accumulated Other Comprehensive Loss
Net of tax
Balance	(88,011)	(51,259)	(51,457)
Other comprehensive (loss) income	(8,372)	(36,752)	198
Balance	(96,383)	(88,011)	(51,259)
Pension and Other Benefits
Pre-tax Amount
Other comprehensive income (loss)	7,092	(5,547)	14,853
Income Tax Benefit (Expense)
Other comprehensive income (loss)	(2,801)	2,193	(5,867)
Net of tax
Balance	(30,237)	(26,883)	(35,869)
Other comprehensive (loss) income	4,291	(3,354)	8,986
Balance	(25,946)	(30,237)	(26,883)
Pension and other benefits, net actuarial gain or loss
Pre-tax Amount
Unrealized net (losses) gains arising during the period	(1,104)	(12,666)	5,322
Reclassification of net losses (gains) to net income	8,625	7,629	9,960
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	436	5,004	(2,102)
Reclassification of net (losses) gains to net income	(3,407)	(3,012)	(3,934)
Net of tax
Unrealized net gains (losses) arising during the period	(668)	(7,662)	3,220
Reclassification of net losses (gains) to net income	5,218	4,617	6,026
Pension and other benefits, net prior service cost or credit
Pre-tax Amount
Reclassification of net losses (gains) to net income	(429)	(429)	(429)
Income Tax Benefit (Expense)
Reclassification of net (losses) gains to net income	170	169	169
Net of tax
Reclassification of net losses (gains) to net income	(259)	(260)	(260)
Pension and other benefits, change due to the Reorganization Transactions
Pre-tax Amount
Unrealized net (losses) gains arising during the period		(81)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period		32
Net of tax
Unrealized net gains (losses) arising during the period		(49)
Investment Securities
Pre-tax Amount
Unrealized net (losses) gains arising during the period	(23,516)	(30,323)	(3,503)
Reclassification of net losses (gains) to net income		(27,277)	(12,321)
Other comprehensive income (loss)	(23,516)	(57,600)	(15,824)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	9,357	11,974	1,384
Reclassification of net (losses) gains to net income		10,774	4,867
Other comprehensive income (loss)	9,357	22,748	6,251
Net of tax
Balance	(59,958)	(25,106)	(15,533)
Unrealized net gains (losses) arising during the period	(14,159)	(18,349)	(2,119)
Reclassification of net losses (gains) to net income		(16,503)	(7,454)
Other comprehensive (loss) income	(14,159)	(34,852)	(9,573)
Balance	(74,117)	(59,958)	(25,106)
Cash Flow Derivative Hedges
Pre-tax Amount
Unrealized net (losses) gains arising during the period	2,473	2,397	1,684
Reclassification of net losses (gains) to net income			(387)
Other comprehensive income (loss)	2,473	2,397	1,297
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(977)	(943)	(665)
Reclassification of net (losses) gains to net income			153
Other comprehensive income (loss)	(977)	(943)	(512)
Net of tax
Balance	2,184	730	(55)
Unrealized net gains (losses) arising during the period	1,496	1,454	1,019
Reclassification of net losses (gains) to net income			(234)
Other comprehensive (loss) income	1,496	1,454	785
Balance	$ 3,680	$ 2,184	$ 730