Fixed Maturity Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Sep. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,375,797	$ 854,391
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	2,415,067	1,274,605
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 2,041,265	$ 1,294,721
Noninvestment Grade Investments at Loss Position
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	45
Aggregate Fair Value	$ 267,607
Gross Unrealized Loss	$ 27,421
Noninvestment Grade Investments at Loss Position | Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	20
Aggregate Fair Value	$ 172,462
Gross Unrealized Loss	$ 20,688
Noninvestment Grade Investments at Loss Position | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	4
Aggregate Fair Value	$ 3,270
Gross Unrealized Loss	$ 30
Noninvestment Grade Investments at Loss Position | Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	14
Aggregate Fair Value	$ 80,945
Gross Unrealized Loss	$ 6,547
Noninvestment Grade Investments at Loss Position | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities | position	7
Aggregate Fair Value	$ 10,930
Gross Unrealized Loss	$ 156