Investments (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) investments	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Available-for-sale Securities, Aggregate fair value, continuous unrealized loss position, 12 months or more	$ 2,975,631,000	$ 362,351,000
Unrealized losses related to marketability of securities or issuers ability to honor redemption obligations	0	0
Investment value deemed to be OTTI	0
Investment securities, aggregate carrying value, pledged as collateral	4,593,198,000	4,548,628,000
Mortgage Backed Securities
Schedule of Investments [Line Items]
Available-for-sale Securities, Aggregate fair value, continuous unrealized loss position, 12 months or more	$ 2,725,933,000	362,351,000
Number of investments in continuous unrealized loss position for more than twelve months | investments	227
U. S. Treasury
Schedule of Investments [Line Items]
Available-for-sale Securities, Aggregate fair value, continuous unrealized loss position, 12 months or more	$ 249,698,000	$ 0
Number of investments in continuous unrealized loss position for more than twelve months | investments	4