Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pre-tax Amount
Balance	$ (111,823)	$ (8,644)	$ (145,472)	$ (84,722)
Other comprehensive income	887	(8,410)	34,536	67,668
Balance	(110,936)	(17,054)	(110,936)	(17,054)
Income Tax Benefit (Expense)
Balance	44,172	3,417	57,461	33,463
Other comprehensive income	(350)	3,322	(13,639)	(26,724)
Balance	43,822	6,739	43,822	6,739
Net of tax
Balance			2,476,485	2,736,941
Other comprehensive income (loss)	537	(5,088)	20,897	40,944
Balance	2,581,858	2,523,963	2,581,858	2,523,963
Accumulated Other Comprehensive Loss
Net of tax
Balance	(67,651)	(5,227)	(88,011)	(51,259)
Other comprehensive income (loss)	537	(5,088)	20,897	40,944
Balance	(67,114)	(10,315)	(67,114)	(10,315)
Pension and other benefits
Pre-tax Amount
Other comprehensive income				(78)
Income Tax Benefit (Expense)
Other comprehensive income				32
Net of tax
Balance	(30,237)	(26,929)	(30,237)	(26,883)
Other comprehensive income (loss)				(46)
Balance	(30,237)	(26,929)	(30,237)	(26,929)
Pension and other benefits, change due to the Reorganization Transactions
Pre-tax Amount
Unrealized net gains (losses) arising during the period				(78)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period				32
Net of tax
Unrealized net gains (losses) arising during the period				(46)
Investment securities
Pre-tax Amount
Unrealized net gains (losses) arising during the period	212	(9,925)	33,005	92,590
Reclassification of net gains to net income		(30)		(25,761)
Other comprehensive income	212	(9,955)	33,005	66,829
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(84)	3,920	(13,035)	(36,573)
Reclassification of net gains to net income		12		10,176
Other comprehensive income	(84)	3,932	(13,035)	(26,397)
Net of tax
Balance	(40,116)	21,349	(59,958)	(25,106)
Unrealized net gains (losses) arising during the period	128	(6,005)	19,970	56,017
Reclassification of gains to net income		(18)		(15,585)
Other comprehensive income (loss)	128	(6,023)	19,970	40,432
Balance	(39,988)	15,326	(39,988)	15,326
Cash flow derivative hedges
Pre-tax Amount
Unrealized net gains (losses) arising during the period	675	1,545	1,531	917
Other comprehensive income	675	1,545	1,531	917
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(266)	(610)	(604)	(359)
Other comprehensive income	(266)	(610)	(604)	(359)
Net of tax
Balance	2,702	353	2,184	730
Unrealized net gains (losses) arising during the period	409	935	927	558
Other comprehensive income (loss)	409	935	927	558
Balance	$ 3,111	$ 1,288	$ 3,111	$ 1,288