Investments (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) investments	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) investments	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Investments [Abstract]
Securities gains (losses)	$ 1,337,000	$ 352,000	$ 4,664,000	$ 17,509,000
Unrealized Losses Related to Marketability of Securities or Issuer's Ability to Honor Redemption Obligations	0		0		$ 0
Fair Value, 12 months or more in unrealized loss position	406,480,000		406,480,000		362,351,000
Unrealized Losses, Greater than 12 months	$ 5,396,000		$ 5,396,000		4,689,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments	55		55
Investment value deemed to be OTTI	$ 0	$ 0
Investment securities, aggregate carrying value, pledged as collateral	$ 4,510,000,000		$ 4,510,000,000		$ 4,550,000,000