Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (54,724)	$ (180,709)
Net change in unrealized gain or loss	165,297	57,243
Reclassification of net income or loss realized and included in earnings	368	4,219
Amortization of unrealized net loss (gain) on securities transferred to HTM	(195)	591
Income tax expense	37,480	13,853
Ending Balance	73,266	(132,509)
Equity Method Investment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(434)
Net change in unrealized gain or loss	(197)	784
Ending Balance	(631)	784
Accumulated Other Comprehensive Loss Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	28,950	(50,125)
Net change in unrealized gain or loss	124,018	46,984
Income tax expense	28,056	10,623
Ending Balance	124,912	(13,764)
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	639	(12,044)
Amortization of unrealized net loss (gain) on securities transferred to HTM	(195)	591
Income tax expense	(44)	134
Ending Balance	488	(11,587)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(101,278)	(110,247)
Reclassification of net income or loss realized and included in earnings	1,232	2,203
Income tax expense	279	498
Ending Balance	(100,325)	(108,542)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	17,399	(8,293)
Net change in unrealized gain or loss	41,476	9,475
Reclassification of net income or loss realized and included in earnings	(864)	2,016
Income tax expense	9,189	2,598
Ending Balance	$ 48,822	$ 600