Investments (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) investments	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Investments [Line Items]
Marketable equity securities	$ 110,107,000		$ 0
Unrealized Losses Related to Marketability of Securities or Issuer's Ability to Honor Redemption Obligations	0		0
Fair Value, 12 months or more in unrealized loss position	2,637,746,000		2,975,631,000
Unrealized Losses, Greater than 12 months	96,056,000		59,470,000
Investment value deemed to be OTTI	0	$ 0
Investment securities, aggregate carrying value, pledged as collateral	3,830,000,000		4,590,000,000
Investment securities available for sale	6,857,740,000		7,180,180,000
Equity Securities
Schedule of Investments [Line Items]
Investment securities available for sale	0		105,208,000
Mortgage Backed Securities
Schedule of Investments [Line Items]
Fair Value, 12 months or more in unrealized loss position	2,535,662,000		2,723,406,000
Unrealized Losses, Greater than 12 months	$ 95,979,000		59,241,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments	224
Investment securities available for sale	$ 5,206,667,000		5,340,756,000
U. S. Treasury
Schedule of Investments [Line Items]
Fair Value, 12 months or more in unrealized loss position	99,925,000		249,698,000
Unrealized Losses, Greater than 12 months	$ 49,000		201,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments	1
Investment securities available for sale	$ 1,553,893,000		$ 1,657,864,000
Financial Instruments ASU cumulative effect adjustment
Schedule of Investments [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 18,700,000