Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	$ 1,220,992	$ 1,461,783
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	9,442	25,415
12 Months or More Fair Value	953,348	165,173
12 Months or More Unrealized Losses	(24,896)	(5,276)
Total Fair Value	2,174,340	1,626,956
Total Unrealized Losses	(34,338)	(30,691)
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	816,678	1,029,116
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	6,710	18,788
12 Months or More Fair Value	717,211	159,046
12 Months or More Unrealized Losses	(20,616)	(4,930)
Total Fair Value	1,533,889	1,188,162
Total Unrealized Losses	(27,326)	(23,718)
State and Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	220,019	211,342
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	1,723	5,064
12 Months or More Fair Value	75,172	3,384
12 Months or More Unrealized Losses	(2,236)	(133)
Total Fair Value	295,191	214,726
Total Unrealized Losses	(3,959)	(5,197)
U.S. Government Agency and Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	184,046	218,811
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	1,006	1,511
12 Months or More Fair Value	155,983	0
12 Months or More Unrealized Losses	(1,756)	0
Total Fair Value	340,029	218,811
Total Unrealized Losses	(2,762)	(1,511)
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	249	251
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	3	1
12 Months or More Fair Value	0	0
12 Months or More Unrealized Losses	0	0
Total Fair Value	249	251
Total Unrealized Losses	(3)	(1)
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months Fair Value	0	2,263
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss, Instant	0	51
12 Months or More Fair Value	4,982	2,743
12 Months or More Unrealized Losses	(288)	(213)
Total Fair Value	4,982	5,006
Total Unrealized Losses	$ (288)	$ (264)