Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax expense at federal statutory rate									$ 123,826	$ 106,208	$ 104,217
SALT expense, net of federal									8,189	6,882	7,563
Tax-exempt interest income, net									(10,826)	(8,917)	(7,117)
SALT DTA adjustments, net of federal									(28,724)	0	(5,785)
Increase in cash surrender value of life insurance									(5,120)	(5,166)	(4,557)
Other, net									(3,536)	(2,684)	(1,289)
Income tax expense	$ 17,029	$ 30,281	$ 29,090	$ 21,951	$ 23,845	$ 24,445	$ 24,599	$ 23,434	$ 98,351	$ 96,323	$ 93,032
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax expense at federal statutory rate, Percent									35.00%	35.00%	35.00%
State and local taxes, net of federal benefit, Percent									2.30%	2.30%	2.50%
Tax-exempt interest income, net, Percent									(3.10%)	(2.90%)	(2.40%)
Decrease in valuation allowance applicable to net state deferred tax assets, net of federal effects, Percent									(8.10%)	0.00%	(1.90%)
Increase in cash surrender value of life insurance, Percent									(1.40%)	(1.70%)	(1.50%)
Other, net, Percent									(1.00%)	(1.00%)	(0.50%)
Effective Tax Rate, Percent									27.80%	31.70%	31.20%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									$ 20,891	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent									5.90%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount									$ (6,349)	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent									(1.80%)	0.00%	0.00%