Securities in an Unrealized Loss Position (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities in continuous unrealized loss position fair value	$ 4,170,376,000	$ 5,160,888,000	$ 4,170,376,000
Securities in continuous unrealized loss position, aggregate losses accumulated In AOCI	113,948,000	$ 99,268,000	113,948,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment				$ 0
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Other		one
Securities in continuous unrealized loss position fair value	22,320,000	$ 22,034,000	22,320,000
Securities in continuous unrealized loss position, aggregate losses accumulated In AOCI	3,353,000	$ 3,639,000	3,353,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	13,400,000
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Other		two
Securities in continuous unrealized loss position fair value	26,470,000	$ 9,091,000	26,470,000
Securities in continuous unrealized loss position, aggregate losses accumulated In AOCI	$ 19,189,000	1,046,000	19,189,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		$ 18,100,000	$ 10,900,000