Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	$ 10,562	$ 276,306
Less than 12 Months Unrealized Losses	(39)	(1,598)
12 Months or More Fair Value	955,713	1,873,565
12 Months or More Unrealized Losses	(12,512)	(54,368)
Total Fair Value	966,275	2,149,871
Total Unrealized Losses	(12,551)	(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	1,165	154,622
Less than 12 Months Unrealized Losses	(6)	(972)
12 Months or More Fair Value	785,380	1,301,387
12 Months or More Unrealized Losses	(11,297)	(42,495)
Total Fair Value	786,545	1,456,009
Total Unrealized Losses	(11,303)	(43,467)
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	8,895	106,292
Less than 12 Months Unrealized Losses	(33)	(581)
12 Months or More Fair Value	105,708	280,496
12 Months or More Unrealized Losses	(1,026)	(7,179)
Total Fair Value	114,603	386,788
Total Unrealized Losses	(1,059)	(7,760)
US Government Corporations and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	502	15,392
Less than 12 Months Unrealized Losses	0	(45)
12 Months or More Fair Value	64,376	291,435
12 Months or More Unrealized Losses	(188)	(4,691)
Total Fair Value	64,878	306,827
Total Unrealized Losses	(188)	(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	249	247
12 Months or More Unrealized Losses	(1)	(3)
Total Fair Value	249	247
Total Unrealized Losses	$ (1)	$ (3)