Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pre-tax Amount
Balance	$ (143,296)	$ (30,369)	$ (145,472)	$ (84,722)
Other comprehensive income	31,473	21,725	33,649	76,078
Balance	(111,823)	(8,644)	(111,823)	(8,644)
Income Tax Benefit (Expense)
Balance	56,604	11,996	57,461	33,463
Other comprehensive income	(12,432)	(8,579)	(13,289)	(30,046)
Balance	44,172	3,417	44,172	3,417
Net of tax
Balance			2,476,485	2,736,941
Other comprehensive income	19,041	13,146	20,360	46,032
Balance	2,552,602	2,501,008	2,552,602	2,501,008
Accumulated Other Comprehensive Loss
Net of tax
Balance	(86,692)	(18,373)	(88,011)	(51,259)
Other comprehensive income	19,041	13,146	20,360	46,032
Balance	(67,651)	(5,227)	(67,651)	(5,227)
Pension and other benefits
Pre-tax Amount
Other comprehensive income		(78)		(78)
Income Tax Benefit (Expense)
Other comprehensive income		32		32
Net of tax
Balance	(30,237)	(26,883)	(30,237)	(26,883)
Other comprehensive income		(46)		(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)		(78)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period		32		32
Net of tax
Unrealized net gains (losses) arising during the period		(46)		(46)
Investment securities
Pre-tax Amount
Unrealized net gains (losses) arising during the period	31,215	21,605	32,793	102,515
Reclassification of net gains to net income		(3)		(25,731)
Other comprehensive income	31,215	21,602	32,793	76,784
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(12,330)	(8,535)	(12,951)	(40,493)
Reclassification of net gains to net income				10,164
Other comprehensive income	(12,330)	(8,535)	(12,951)	(30,329)
Net of tax
Balance	(59,001)	8,282	(59,958)	(25,106)
Unrealized net gains (losses) arising during the period	18,885	13,070	19,842	62,022
Reclassification of gains to net income		(3)		(15,567)
Other comprehensive income	18,885	13,067	19,842	46,455
Balance	(40,116)	21,349	(40,116)	21,349
Cash flow derivative hedges
Pre-tax Amount
Unrealized net gains (losses) arising during the period	258	201	856	(628)
Other comprehensive income	258	201	856	(628)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(102)	(76)	(338)	251
Other comprehensive income	(102)	(76)	(338)	251
Net of tax
Balance	2,546	228	2,184	730
Unrealized net gains (losses) arising during the period	156	125	518	(377)
Other comprehensive income	156	125	518	(377)
Balance	$ 2,702	$ 353	$ 2,702	$ 353