Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	$ 1,489,258	$ 1,220,992
Less than 12 Months Unrealized Losses	(28,018)	(9,442)
12 Months or More Fair Value	898,484	948,366
12 Months or More Unrealized Losses	(35,799)	(24,608)
Total Fair Value	2,387,742	2,169,358
Total Unrealized Losses	(63,817)	(34,050)
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	921,993	816,678
Less than 12 Months Unrealized Losses	(19,073)	(6,710)
12 Months or More Fair Value	681,577	717,211
12 Months or More Unrealized Losses	(29,556)	(20,616)
Total Fair Value	1,603,570	1,533,889
Total Unrealized Losses	(48,629)	(27,326)
State and Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	315,392	220,019
Less than 12 Months Unrealized Losses	(4,972)	(1,723)
12 Months or More Fair Value	76,452	75,172
12 Months or More Unrealized Losses	(3,965)	(2,236)
Total Fair Value	391,844	295,191
Total Unrealized Losses	(8,937)	(3,959)
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	251,626	184,046
Less than 12 Months Unrealized Losses	(3,969)	(1,006)
12 Months or More Fair Value	140,455	155,983
12 Months or More Unrealized Losses	(2,278)	(1,756)
Total Fair Value	392,081	340,029
Total Unrealized Losses	(6,247)	(2,762)
US Government Securities [Member]
Schedule of Available-for-sale Securities [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)