Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pre-tax Amount
Balance at beginning of period	$ (84,722)	$ (85,048)	$ (75,640)
Other comprehensive income	(60,750)	326	(9,408)
Balance at end of period	(145,472)	(84,722)	(85,048)
Income Tax Benefit (Expense)
Balance at beginning of period	33,463	33,591	29,875
Other comprehensive income	23,998	(128)	3,716
Balance at end of period	57,461	33,463	33,591
Net of tax
Balance	2,736,941	2,675,040	2,651,111
Other comprehensive (loss) income	(36,752)	198	(5,692)
Balance	2,476,485	2,736,941	2,675,040
Accumulated Other Comprehensive Loss
Net of tax
Balance	(51,259)	(51,457)	(45,765)
Other comprehensive (loss) income	(36,752)	198	(5,692)
Balance	(88,011)	(51,259)	(51,457)
Pension and other benefits
Pre-tax Amount
Other comprehensive income	(5,547)	14,853	(27,518)
Income Tax Benefit (Expense)
Other comprehensive income	2,193	(5,867)	10,870
Net of tax
Balance	(26,883)	(35,869)	(19,221)
Other comprehensive (loss) income	(3,354)	8,986	(16,648)
Balance	(30,237)	(26,883)	(35,869)
Pension and other benefits, net actuarial gain or loss
Pre-tax Amount
Unrealized net gains (losses) arising during the period	(12,666)	5,322	(34,877)
Reclassification of net gains (loss) to net income	7,629	9,960	5,163
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	5,004	(2,102)	13,776
Reclassification of net gains (losses) to net income	(3,012)	(3,934)	(2,039)
Net of tax
Unrealized net gains (losses) arising during the period	(7,662)	3,220	(21,101)
Reclassification of net gains to net income	4,617	6,026	3,124
Pension and other benefits, net prior service cost or credit
Pre-tax Amount
Unrealized net gains (losses) arising during the period			2,196
Reclassification of net gains (loss) to net income	(429)	(429)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period			(867)
Reclassification of net gains (losses) to net income	169	169
Net of tax
Unrealized net gains (losses) arising during the period			1,329
Reclassification of net gains to net income	(260)	(260)
Pension and other benefits, change due to the Reorganization Transactions
Pre-tax Amount
Unrealized net gains (losses) 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 gains (losses) arising during the period	(30,323)	(3,503)	(1,308)
Reclassification of net gains (loss) to net income	(27,277)	(12,321)	20,822
Other comprehensive income	(57,600)	(15,824)	19,514
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	11,974	1,384	517
Reclassification of net gains (losses) to net income	10,774	4,867	(8,225)
Other comprehensive income	22,748	6,251	(7,708)
Net of tax
Balance	(25,106)	(15,533)	(27,339)
Unrealized net gains (losses) arising during the period	(18,349)	(2,119)	(791)
Reclassification of net gains to net income	(16,503)	(7,454)	12,597
Other comprehensive (loss) income	(34,852)	(9,573)	11,806
Balance	(59,958)	(25,106)	(15,533)
Cash flow derivative hedges
Pre-tax Amount
Unrealized net gains (losses) arising during the period	2,397	1,684	(1,404)
Reclassification of net gains (loss) to net income		(387)
Other comprehensive income	2,397	1,297	(1,404)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(943)	(665)	554
Reclassification of net gains (losses) to net income		153
Other comprehensive income	(943)	(512)	554
Net of tax
Balance	730	(55)	795
Unrealized net gains (losses) arising during the period	1,454	1,019	(850)
Reclassification of net gains to net income		(234)
Other comprehensive (loss) income	1,454	785	(850)
Balance	$ 2,184	$ 730	$ (55)