Investments (Unrealized Losses) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Unrealized Losses Related to Marketability of Securities or Issuer's Ability to Honor Redemption Obligations	$ 0	$ 0
Investment securities available for sale:
Fair Value, Less than 12 months	3,880,315,000	3,752,642,000
Unrealized Losses, Less than 12 months	62,816,000	21,458,000
Fair Value, 12 months or more	2,637,746,000	2,975,631,000
Unrealized Losses, Greater than 12 months	96,056,000	59,470,000
Fair Value, Total	6,518,061,000	6,728,273,000
Unrealized Losses, Total	158,872,000	80,928,000
U. S. Treasury
Investment securities available for sale:
Fair Value, Less than 12 months	1,453,968,000	1,408,166,000
Unrealized Losses, Less than 12 months	4,049,000	345,000
Fair Value, 12 months or more	99,925,000	249,698,000
Unrealized Losses, Greater than 12 months	49,000	201,000
Fair Value, Total	1,553,893,000	1,657,864,000
Unrealized Losses, Total	4,098,000	546,000
Government Agency
Investment securities available for sale:
Fair Value, Less than 12 months	12,059,000	848,000
Unrealized Losses, Less than 12 months	81,000	12,000
Fair Value, 12 months or more	2,159,000	2,527,000
Unrealized Losses, Greater than 12 months	28,000	28,000
Fair Value, Total	14,218,000	3,375,000
Unrealized Losses, Total	109,000	40,000
Mortgage Backed Securities
Investment securities available for sale:
Fair Value, Less than 12 months	2,404,338,000	2,333,254,000
Unrealized Losses, Less than 12 months	58,606,000	20,911,000
Fair Value, 12 months or more	2,535,662,000	2,723,406,000
Unrealized Losses, Greater than 12 months	95,979,000	59,241,000
Fair Value, Total	4,940,000,000	5,056,660,000
Unrealized Losses, Total	154,585,000	80,152,000
Corporate bonds
Investment securities available for sale:
Fair Value, Less than 12 months		5,025,000
Unrealized Losses, Less than 12 months		8,000
Fair Value, 12 months or more		0
Unrealized Losses, Greater than 12 months		0
Fair Value, Total		5,025,000
Unrealized Losses, Total		8,000
Equity Securities
Investment securities available for sale:
Unrealized Losses, Total		0
Corporate Bonds
Investment securities available for sale:
Fair Value, Less than 12 months	9,950,000
Unrealized Losses, Less than 12 months	80,000
Fair Value, 12 months or more	0
Unrealized Losses, Greater than 12 months	0
Fair Value, Total	9,950,000
Unrealized Losses, Total	80,000	8,000
Other
Investment securities available for sale:
Fair Value, Less than 12 months		5,349,000
Unrealized Losses, Less than 12 months		182,000
Fair Value, 12 months or more		0
Unrealized Losses, Greater than 12 months		0
Fair Value, Total		5,349,000
Unrealized Losses, Total	$ 0	$ 182,000