Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Sep. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 3,902,366	$ 5,160,888
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	748,957	734,969
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	708,930	1,100,392
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,247,612	$ 1,212,529
Noninvestment Grade Investments at Loss Position
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	21
Aggregate Fair Value	$ 65,450
Gross Unrealized Loss	$ 938
Noninvestment Grade Investments at Loss Position | Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	9
Aggregate Fair Value	$ 55,292
Gross Unrealized Loss	$ 462
Noninvestment Grade Investments at Loss Position | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	6
Aggregate Fair Value	$ 5,975
Gross Unrealized Loss	$ 150
Noninvestment Grade Investments at Loss Position | Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	3
Aggregate Fair Value	$ 2,852
Gross Unrealized Loss	$ 211
Noninvestment Grade Investments at Loss Position | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	3
Aggregate Fair Value	$ 1,331
Gross Unrealized Loss	$ 115