Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning Balance			$ 1,015,974	$ 797,133
Other comprehensive income (loss) before reclassifications	$ (1,316)	$ 1,214	2,793	1,545
Amounts reclassified from accumulated other comprehensive income	1,872	704	1,097	1,722
Other comprehensive income, net of tax	556	1,918	3,890	3,267
Ending Balance	2,563,666	998,363	2,563,666	998,363
Unrealized gains (losses) on securities available-for-sale, net of tax [Member]
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning Balance	2,196	(274)	(1,888)	(210)
Other comprehensive income (loss) before reclassifications	(805)	1,214	3,304	1,545
Amounts reclassified from accumulated other comprehensive income	(10)	(3)	(35)	(398)
Other comprehensive income, net of tax	(815)	1,211	3,269	1,147
Ending Balance	1,381	937	1,381	937
Defined Benefit Pension Plans [Member]
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning Balance	(27,894)	(30,830)	(27,144)	(32,243)
Other comprehensive income (loss) before reclassifications	(511)	0	(511)	0
Amounts reclassified from accumulated other comprehensive income	1,882	707	1,132	2,120
Other comprehensive income, net of tax	1,371	707	621	2,120
Ending Balance	(26,523)	(30,123)	(26,523)	(30,123)
Total [Member]
Components of accumulated other comprehensive income (loss), net of related tax benefit/expense
Beginning Balance	(25,698)	(31,104)	(29,032)	(32,453)
Ending Balance	$ (25,142)	$ (29,186)	$ (25,142)	$ (29,186)