Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax expense at federal statutory rate									$ 105,111	$ 102,095	$ 89,799
State and local income taxes, net of federal benefit									7,613	3,390	2,802
Tax-exempt interest income, net									(7,117)	(7,335)	(8,517)
Decrease in valuation allowance applicable to net state deferred tax assets, net of federal effects									(5,785)	0	0
Increase in cash surrender value of life insurance									(4,557)	(4,612)	(4,819)
Other, net									(1,289)	(1,565)	(2,152)
Income tax expense	$ 24,146	$ 25,075	$ 20,663	$ 24,092	$ 23,753	$ 23,824	$ 23,159	$ 21,237	$ 93,976	$ 91,973	$ 77,113
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent									35.00%	35.00%	35.00%
State and local taxes, net of federal benefit, Percent									2.50%	1.20%	1.10%
Tax-exempt interest income, net, Percent									(2.40%)	(2.50%)	(3.30%)
Decrease in valuation allowance applicable to net state deferred tax assets, net of federal effects, Percent									(1.90%)	0.00%	0.00%
Increase in cash surrender value of life insurance, Percent									(1.50%)	(1.60%)	(1.90%)
Other, net, Percent									(0.40%)	(0.60%)	(0.80%)
Effective Tax Rate, Percent									31.30%	31.50%	30.10%