Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (180,709)	$ (134,402)
Net change in unrealized gain or loss	57,243	(62,244)
Reclassification of net loss realized and included in earnings	4,219	1,796
Amortization of unrealized net loss on securities transferred to HTM	591	755
Income tax expense (benefit)	13,853	(13,546)
Ending Balance	(132,509)	(180,549)
Equity Method Investment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net change in unrealized gain or loss	784
Ending Balance	784
Accumulated Other Comprehensive Loss Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(50,125)	(29,512)
Net change in unrealized gain or loss	46,984	(55,114)
Income tax expense (benefit)	10,623	(12,508)
Ending Balance	(13,764)	(72,118)
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(12,044)	(14,585)
Amortization of unrealized net loss on securities transferred to HTM	591	755
Income tax expense (benefit)	134	171
Ending Balance	(11,587)	(14,001)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(110,247)	(79,078)
Reclassification of net loss realized and included in earnings	2,203	1,177
Income tax expense (benefit)	498	267
Ending Balance	(108,542)	(78,168)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(8,293)	(11,227)
Net change in unrealized gain or loss	9,475	(7,130)
Reclassification of net loss realized and included in earnings	2,016	619
Income tax expense (benefit)	2,598	(1,476)
Ending Balance	$ 600	$ (16,262)