Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	$ 1,015,974	$ 797,133	$ 696,500
Other comprehensive loss before reclassifications	(12,399)	1,174	(13,353)
Amounts reclassified from accumulated other comprehensive income	1,395	2,247	1,420
Net current period other comprehensive income (loss)	(11,004)	3,421	(11,933)
Ending balance	2,581,526	1,015,974	797,133
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	(1,888)	(210)	(2,480)
Other comprehensive loss before reclassifications	(12,170)	(1,269)	2,270
Amounts reclassified from accumulated other comprehensive income	(84)	(409)	0
Net current period other comprehensive income (loss)	(12,254)	(1,678)	2,270
Ending balance	(14,142)	(1,888)	(210)
Defined Benefit Pension Plans
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(27,144)	(32,243)	(18,040)
Other comprehensive loss before reclassifications	(229)	2,443	(15,623)
Amounts reclassified from accumulated other comprehensive income	1,479	2,656	1,420
Net current period other comprehensive income (loss)	1,250	5,099	(14,203)
Ending balance	(25,894)	(27,144)	(32,243)
Total
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning balance	(29,032)	(32,453)	(20,520)
Ending balance	$ (40,036)	$ (29,032)	$ (32,453)