Securities In An Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Mar. 31, 2018 USD ($) position	Dec. 31, 2017 USD ($)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,117,131	$ 854,391
Corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	2,081,663	1,274,605
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 1,432,006	$ 1,294,721
Noninvestment Grade Investments at Loss Position
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	27
Aggregate Fair Value	$ 125,885
Gross Unrealized Loss	$ 5,428
Noninvestment Grade Investments at Loss Position | Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	8
Aggregate Fair Value	$ 30,997
Gross Unrealized Loss	$ 739
Noninvestment Grade Investments at Loss Position | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	5
Aggregate Fair Value	$ 4,532
Gross Unrealized Loss	$ 117
Noninvestment Grade Investments at Loss Position | Corporate
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	10
Aggregate Fair Value	$ 86,345
Gross Unrealized Loss	$ 4,429
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	$ 370
Gross Unrealized Loss	$ 140
Noninvestment Grade Investments at Loss Position | State and municipal
Schedule of Available-for-sale Securities [Line Items]
Number of Securities	1
Aggregate Fair Value	$ 3,641
Gross Unrealized Loss	$ 3