Securities In An Unrealized Loss Position (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 3,902,366,000		$ 3,902,366,000		$ 5,160,888,000
Gross unrealized losses	45,881,000		45,881,000		99,268,000
Other-than-temporary impairments	0	$ 0	0	$ 18,114,000
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments			0	0
Common stock
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments			0	$ 18,100,000
Common stock
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	$ 3,500,000		$ 3,500,000
Noninvestment Grade Investments
Schedule of Available-for-sale Securities [Line Items]
Number of securities | shares	1		1
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 23,957,000		$ 23,957,000		22,034,000
Gross unrealized losses	1,718,000		1,718,000		3,639,000
Common stock
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	14,065,000		14,065,000		9,091,000
Gross unrealized losses	$ 3,486,000		3,486,000		$ 1,046,000
Other-than-temporary impairment, available for sale securities			$ 14,100,000