Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	$ 1,718,571	$ 1,220,992
Less than 12 Months Unrealized Losses	(40,068)	(9,442)
12 Months or More Fair Value	1,051,352	948,366
12 Months or More Unrealized Losses	(50,463)	(24,608)
Total Fair Value	2,769,923	2,169,358
Total Unrealized Losses	(90,531)	(34,050)
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	1,084,529	816,678
Less than 12 Months Unrealized Losses	(26,966)	(6,710)
12 Months or More Fair Value	834,985	717,211
12 Months or More Unrealized Losses	(42,233)	(20,616)
Total Fair Value	1,919,514	1,533,889
Total Unrealized Losses	(69,199)	(27,326)
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	333,523	220,019
Less than 12 Months Unrealized Losses	(6,833)	(1,723)
12 Months or More Fair Value	96,018	75,172
12 Months or More Unrealized Losses	(5,408)	(2,236)
Total Fair Value	429,541	295,191
Total Unrealized Losses	(12,241)	(3,959)
US Government Corporations and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	300,519	184,046
Less than 12 Months Unrealized Losses	(6,269)	(1,006)
12 Months or More Fair Value	120,102	155,983
12 Months or More Unrealized Losses	(2,818)	(1,756)
Total Fair Value	420,621	340,029
Total Unrealized Losses	(9,087)	(2,762)
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	0	249
Less than 12 Months Unrealized Losses	0	(3)
12 Months or More Fair Value	247	0
12 Months or More Unrealized Losses	(4)	0
Total Fair Value	247	249
Total Unrealized Losses	$ (4)	$ (3)