Accumulated Other Comprehensive Loss - Components of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 2,533,165
Beginning balance after reclassifications	$ 3,277,312		2,533,165
Amounts reclassified from other comprehensive (loss) income	773	$ 1,363	2,453	$ 2,823
Ending balance	3,277,312		3,277,312
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(32,865)		(12,004)
Beginning balance after reclassifications	(32,865)		(12,004)		$ (12,484)
Other comprehensive (loss) income before reclassifications	(6,461)		(27,390)
Amounts reclassified from other comprehensive (loss) income	30		578
Other comprehensive (loss) income, net	(6,431)		(26,812)
Ending balance	(39,296)		(39,296)
Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(664)		(380)
Beginning balance after reclassifications	(664)		(380)		(380)
Other comprehensive (loss) income before reclassifications	212		(56)
Amounts reclassified from other comprehensive (loss) income	12		(4)
Other comprehensive (loss) income, net	224		(60)
Ending balance	(440)		(440)
Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(5,403)		(8,338)
Beginning balance after reclassifications	(5,403)		(8,338)		(8,399)
Other comprehensive (loss) income before reclassifications	455		2,415
Amounts reclassified from other comprehensive (loss) income	619		1,655
Other comprehensive (loss) income, net	1,074		4,070
Ending balance	(4,329)		(4,329)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(25,171)		(25,283)
Beginning balance after reclassifications	(25,171)		(25,283)		(25,283)
Other comprehensive (loss) income before reclassifications	0		0
Amounts reclassified from other comprehensive (loss) income	112	$ 59	224	$ 118
Other comprehensive (loss) income, net	112		224
Ending balance	(25,059)		(25,059)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(64,103)		(46,005)
Beginning balance after reclassifications	(64,103)		(46,005)		$ (46,546)
Other comprehensive (loss) income before reclassifications	(5,794)		(25,031)
Amounts reclassified from other comprehensive (loss) income	773		2,453
Other comprehensive (loss) income, net	(5,021)		(22,578)
Ending balance	$ (69,124)		(69,124)
Accounting Standards Update 2016-01 | Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			(480)
Accounting Standards Update 2016-01 | Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2016-01 | Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2016-01 | Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2016-01 | Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			(480)
Accounting Standards Update 2017-02 | Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2017-02 | Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2017-02 | Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			(61)
Accounting Standards Update 2017-02 | Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			0
Accounting Standards Update 2017-02 | Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle			$ (61)