Securities - Securities in Continuous Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Available for Sale, Estimated Fair Value, Less than 12 Months	$ 37,938	$ 50,962
Available for Sale, Unrealized Losses, Less than 12 Months	(107)	(197)
Available for Sale, Estimated Fair Value, 12 Months or More	1,947	4,187
Available for Sale, Unrealized Losses, 12 Months or More	(25)	(40)
Available for Sale, Estimated Fair Value, Total	39,885	55,149
Available for Sale, Unrealized Losses, Total	(132)	(237)
Held to Maturity, Estimated Fair Value, Less than 12 Months	175,108	1,113,216
Held to Maturity, Unrealized Losses, Less than 12 Months	(444)	(8,805)
Held to Maturity, Estimated Fair Value, 12 Months or More	4,307,088	6,937,459
Held to Maturity, Unrealized Losses, 12 Months or More	(44,926)	(244,009)
Held to Maturity, Estimated Fair Value, Total	4,482,196	8,050,675
Held to Maturity, Unrealized Losses, Total	(45,370)	(252,814)
Collateralized Mortgage Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for Sale, Estimated Fair Value, Less than 12 Months	37,871	12
Available for Sale, Unrealized Losses, Less than 12 Months	(106)
Available for Sale, Estimated Fair Value, 12 Months or More	1,899	2,096
Available for Sale, Unrealized Losses, 12 Months or More	(24)	(37)
Available for Sale, Estimated Fair Value, Total	39,770	2,108
Available for Sale, Unrealized Losses, Total	(130)	(37)
Held to Maturity, Estimated Fair Value, Less than 12 Months	79
Held to Maturity, Unrealized Losses, Less than 12 Months	(1)
Held to Maturity, Estimated Fair Value, 12 Months or More		292
Held to Maturity, Unrealized Losses, 12 Months or More		(2)
Held to Maturity, Estimated Fair Value, Total	79	292
Held to Maturity, Unrealized Losses, Total	(1)	(2)
Collateralized Mortgage Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available for Sale, Estimated Fair Value, Less than 12 Months	67	50,950
Available for Sale, Unrealized Losses, Less than 12 Months	(1)	(197)
Available for Sale, Estimated Fair Value, 12 Months or More	48	2,091
Available for Sale, Unrealized Losses, 12 Months or More	(1)	(3)
Available for Sale, Estimated Fair Value, Total	115	53,041
Available for Sale, Unrealized Losses, Total	(2)	(200)
Held to Maturity, Estimated Fair Value, Less than 12 Months	131,659	1,003,089
Held to Maturity, Unrealized Losses, Less than 12 Months	(356)	(8,401)
Held to Maturity, Estimated Fair Value, 12 Months or More	4,271,546	6,873,948
Held to Maturity, Unrealized Losses, 12 Months or More	(44,866)	(243,534)
Held to Maturity, Estimated Fair Value, Total	4,403,205	7,877,037
Held to Maturity, Unrealized Losses, Total	(45,222)	(251,935)
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Held to Maturity, Estimated Fair Value, Less than 12 Months		20,720
Held to Maturity, Unrealized Losses, Less than 12 Months		(76)
Held to Maturity, Estimated Fair Value, 12 Months or More	8,990	4,957
Held to Maturity, Unrealized Losses, 12 Months or More	(3)	(24)
Held to Maturity, Estimated Fair Value, Total	8,990	25,677
Held to Maturity, Unrealized Losses, Total	(3)	(100)
US States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Held to Maturity, Estimated Fair Value, Less than 12 Months	43,370	89,407
Held to Maturity, Unrealized Losses, Less than 12 Months	(87)	(328)
Held to Maturity, Estimated Fair Value, 12 Months or More	26,552	58,262
Held to Maturity, Unrealized Losses, 12 Months or More	(57)	(449)
Held to Maturity, Estimated Fair Value, Total	69,922	147,669
Held to Maturity, Unrealized Losses, Total	$ (144)	$ (777)