Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	$ 1,474,517	$ 1,220,992
Less than 12 Months Unrealized Losses	(32,847)	(9,442)
12 Months or More Fair Value	909,144	948,366
12 Months or More Unrealized Losses	(40,615)	(24,608)
Total Fair Value	2,383,661	2,169,358
Total Unrealized Losses	(73,462)	(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	906,188	816,678
Less than 12 Months Unrealized Losses	(22,365)	(6,710)
12 Months or More Fair Value	722,422	717,211
12 Months or More Unrealized Losses	(33,989)	(20,616)
Total Fair Value	1,628,610	1,533,889
Total Unrealized Losses	(56,354)	(27,326)
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	306,897	220,019
Less than 12 Months Unrealized Losses	(5,536)	(1,723)
12 Months or More Fair Value	76,988	75,172
12 Months or More Unrealized Losses	(4,340)	(2,236)
Total Fair Value	383,885	295,191
Total Unrealized Losses	(9,876)	(3,959)
US Government Corporations and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	261,185	184,046
Less than 12 Months Unrealized Losses	(4,942)	(1,006)
12 Months or More Fair Value	109,734	155,983
12 Months or More Unrealized Losses	(2,286)	(1,756)
Total Fair Value	370,919	340,029
Total Unrealized Losses	(7,228)	(2,762)
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	247	249
Less than 12 Months Unrealized Losses	(4)	(3)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Losses	0	0
Total Fair Value	247	249
Total Unrealized Losses	$ (4)	$ (3)