Investments (Narrative) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) investments	Dec. 31, 2015 USD ($)
Investments [Abstract]
Available-for-sale Securities, Aggregate fair value, continuous unrealized loss position, 12 months or more	$ 362,351,000	$ 280,126,000
Number of investments in continuous unrealized loss position for more than twelve months | investments	51
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	$ 4,548,628,000	$ 4,726,446,000