Stockholders' Equity (Components of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 102,726		$ (54,724)	$ (180,709)
Net change in unrealized gain or loss	51,532	$ 69,225	216,829	126,468
Reclassification of net income or loss realized and included in earnings	(2,885)	3,984	(2,517)	8,203
Amortization of unrealized net loss (gain) on securities transferred to HTM	(94)	890	(289)	1,481
Valuation adjustment to employee benefit plans			(10,251)
Income tax expense	8,842	16,793	46,322	30,646
Ending Balance	102,726	(75,203)	102,726	(75,203)
Equity Method Investment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,418)		(434)
Net change in unrealized gain or loss			(984)	611
Ending Balance		611		611
Accumulated Other Comprehensive Loss Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	161,524		28,950	(50,125)
Net change in unrealized gain or loss			171,333	96,514
Income tax expense			38,759	21,822
Ending Balance		24,567		24,567
Held to Maturity Securities Transferred from AFS [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	415		639	(12,044)
Amortization of unrealized net loss (gain) on securities transferred to HTM			(289)	1,481
Income tax expense			(65)	335
Ending Balance		(10,898)		(10,898)
Employee Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(106,932)		(101,278)	(110,247)
Reclassification of net income or loss realized and included in earnings			2,944	4,527
Valuation adjustment to employee benefit plans			(10,251)
Income tax expense			(1,653)	1,023
Ending Balance		(106,743)		(106,743)
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 49,137		17,399	(8,293)
Net change in unrealized gain or loss			46,480	29,343
Reclassification of net income or loss realized and included in earnings			(5,461)	3,676
Income tax expense			$ 9,281	7,466
Ending Balance		$ 17,260		$ 17,260