Investments (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) investments	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) investments	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Investments [Abstract]
Securities gains (losses)	$ 3,351,000	$ 12,529,000	$ 3,327,000	$ 17,157,000
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	339,826,000		339,826,000		362,351,000
Unrealized Losses, Greater than 12 months	$ 4,491,000		$ 4,491,000		4,689,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments	52		52
Investment value deemed to be OTTI	$ 0	$ 0
Investment securities, aggregate carrying value, pledged as collateral	$ 4,710,000,000		$ 4,710,000,000		$ 4,550,000,000