Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	$ 5,191	$ 276,306
Less than 12 Months Unrealized Losses	(8)	(1,598)
12 Months or More Fair Value	1,713,893	1,873,565
12 Months or More Unrealized Losses	(30,595)	(54,368)
Total Fair Value	1,719,084	2,149,871
Total Unrealized Losses	(30,603)	(55,966)
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	1,879	154,622
Less than 12 Months Unrealized Losses	(7)	(972)
12 Months or More Fair Value	1,250,289	1,301,387
12 Months or More Unrealized Losses	(26,093)	(42,495)
Total Fair Value	1,252,168	1,456,009
Total Unrealized Losses	(26,100)	(43,467)
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	3,312	106,292
Less than 12 Months Unrealized Losses	(1)	(581)
12 Months or More Fair Value	209,604	280,496
12 Months or More Unrealized Losses	(2,560)	(7,179)
Total Fair Value	212,916	386,788
Total Unrealized Losses	(2,561)	(7,760)
US Government Corporations and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	0	15,392
Less than 12 Months Unrealized Losses	0	(45)
12 Months or More Fair Value	253,752	291,435
12 Months or More Unrealized Losses	(1,940)	(4,691)
Total Fair Value	253,752	306,827
Total Unrealized Losses	(1,940)	(4,736)
US Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Losses	0	0
12 Months or More Fair Value	248	247
12 Months or More Unrealized Losses	(2)	(3)
Total Fair Value	248	247
Total Unrealized Losses	$ (2)	$ (3)