Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 3,127	$ 1,379	4,908	4,108
Amounts reclassified from accumulated other comprehensive income	709	(387)	999	(774)
Other comprehensive income, net of tax	3,836	992	5,907	3,334
Ending balance	2,639,442	1,050,299	2,639,442	1,050,299
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	(12,420)	829	(14,142)	(1,888)
Other comprehensive income before reclassifications	3,593	1,379	5,374	4,108
Amounts reclassified from accumulated other comprehensive income	(50)	(12)	(109)	(24)
Other comprehensive income, net of tax	3,543	1,367	5,265	4,084
Ending balance	(8,877)	2,196	(8,877)	2,196
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(25,545)	(27,519)	(25,894)	(27,144)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	350	(375)	699	(750)
Other comprehensive income, net of tax	350	(375)	699	(750)
Ending balance	(25,195)	(27,894)	(25,195)	(27,894)
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	0	0	0	0
Other comprehensive income before reclassifications	(466)	0	(466)	0
Amounts reclassified from accumulated other comprehensive income	409	0	409	0
Other comprehensive income, net of tax	(57)	0	(57)	0
Ending balance	(57)	0	(57)	0
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(37,965)	(26,690)	(40,036)	(29,032)
Ending balance	$ (34,129)	$ (25,698)	$ (34,129)	$ (25,698)