Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance			$ 2,581,526	$ 1,015,974
Other comprehensive income before reclassifications	$ 8,720	$ (1,316)	13,627	2,793
Amounts reclassified from accumulated other comprehensive income	579	1,872	1,579	1,097
Other comprehensive income, net of tax	9,299	556	15,206	3,890
Ending balance	2,673,089	2,563,666	2,673,089	2,563,666
Unrealized gains (losses) on securities available-for-sale, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(8,877)	2,196	(14,142)	(1,888)
Other comprehensive income before reclassifications	895	(805)	6,268	3,304
Amounts reclassified from accumulated other comprehensive income	(1)	(10)	(109)	(35)
Other comprehensive income, net of tax	894	(815)	6,159	3,269
Ending balance	(7,983)	1,381	(7,983)	1,381
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(25,195)	(27,894)	(25,894)	(27,144)
Other comprehensive income before reclassifications	7,305	(511)	7,305	(511)
Amounts reclassified from accumulated other comprehensive income	349	1,882	1,048	1,132
Other comprehensive income, net of tax	7,654	1,371	8,353	621
Ending balance	(17,541)	(26,523)	(17,541)	(26,523)
Unrealized gains (losses) on cash flow hedges, net of tax
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(57)	0	0	0
Other comprehensive income before reclassifications	520	0	54	0
Amounts reclassified from accumulated other comprehensive income	231	0	640	0
Other comprehensive income, net of tax	751	0	694	0
Ending balance	694	0	694	0
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(34,129)	(25,698)	(40,036)	(29,032)
Ending balance	$ (24,830)	$ (25,142)	$ (24,830)	$ (25,142)