Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		$ 2,581,526	$ 1,015,974	$ 797,133
Other comprehensive loss before reclassifications		12,054	(12,399)	1,174
Amounts reclassified from accumulated other comprehensive income		7,002	1,395	2,247
Net current period other comprehensive income (loss)		19,056	(11,004)	3,421
Reclassification of certain income tax effects	[1]	0
Ending balance		2,668,749	2,581,526	1,015,974
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(40,036)	(29,032)	(32,453)
Reclassification of certain income tax effects	[1]	4,518
Ending balance		(25,498)	(40,036)	(29,032)
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)	(210)
Other comprehensive loss before reclassifications		826	(12,170)	(1,269)
Amounts reclassified from accumulated other comprehensive income		4,802	(84)	(409)
Net current period other comprehensive income (loss)		5,628	(12,254)	(1,678)
Reclassification of certain income tax effects		1,834
Ending balance		(10,348)	(14,142)	(1,888)
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance		(25,894)	(27,144)	(32,243)
Other comprehensive loss before reclassifications		8,457	(229)	2,443
Amounts reclassified from accumulated other comprehensive income		1,139	1,479	2,656
Net current period other comprehensive income (loss)		9,596	1,250	5,099
Reclassification of certain income tax effects		3,510
Ending balance		(19,808)	(25,894)	(27,144)
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
Other comprehensive loss before reclassifications		2,771	0	0
Amounts reclassified from accumulated other comprehensive income		1,061	0	0
Net current period other comprehensive income (loss)		3,832	0	0
Reclassification of certain income tax effects		(826)
Ending balance		4,658	$ 0	$ 0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Reclassification of certain income tax effects		$ 4,518

[1] The reclassification from accumulated other comprehensive income (loss) to retained earnings was due to the early adoption of ASU 2018-02, refer to Note 1 for further details on the adoption.