Securities in an Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 4,981,337	$ 5,160,888
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	269,963	238,066
Total corporate
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	1,274,605	1,100,392
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	854,391	734,969
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	1,294,721	1,212,529
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 4,939,629	$ 5,129,763
Unrealized Loss Less than 5 Million | Foreign government
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	11
Aggregate Fair Value	$ 96,741
Gross Unrealized Loss	$ 1,197
Unrealized Loss Less than 5 Million | Total corporate
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	7
Aggregate Fair Value	$ 54,590
Gross Unrealized Loss	$ 2,725
Unrealized Loss Less than 5 Million | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	6
Aggregate Fair Value	$ 5,368
Gross Unrealized Loss	$ 138
Unrealized Loss Less than 5 Million | State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	1
Aggregate Fair Value	$ 3,662
Gross Unrealized Loss	$ 1
Unrealized Loss Less than 5 Million | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	3
Aggregate Fair Value	$ 441
Gross Unrealized Loss	$ 116
Unrealized Loss Less than 5 Million | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	28
Aggregate Fair Value	$ 160,802
Gross Unrealized Loss	$ 4,177