Accumulated Other Comprehensive Loss - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	1,780	2,729
Amounts reclassified from accumulated other comprehensive income	291	(387)
Other comprehensive income, net of tax	2,071	2,342
Ending balance	2,600,051	1,032,291
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	(14,142)	(1,888)
Other comprehensive income before reclassifications	1,780	2,729
Amounts reclassified from accumulated other comprehensive income	(58)	(12)
Other comprehensive income, net of tax	1,722	2,717
Ending balance	(12,420)	829
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(25,894)	(27,144)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	349	(375)
Other comprehensive income, net of tax	349	(375)
Ending balance	(25,545)	(27,519)
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(40,036)	(29,032)
Ending balance	$ (37,965)	$ (26,690)