Securities in an Unrealized Loss Position (Non-Investment Grade Fixed Maturity Securities) (Details) $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 5,160,888	$ 4,170,376
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	22,034	22,320
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	734,969	566,793
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	1,212,529	1,281,545
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	1,100,392	1,309,737
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 5,129,763	$ 4,121,586
Unrealized Loss Less than 5 Million | Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	11
Aggregate Fair Value	$ 22,987
Gross Unrealized Loss	$ 1,106
Unrealized Loss Less than 5 Million | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	4
Aggregate Fair Value	$ 1,256
Gross Unrealized Loss	$ 362
Unrealized Loss Less than 5 Million | Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	10
Aggregate Fair Value	$ 78,462
Gross Unrealized Loss	$ 1,370
Unrealized Loss Less than 5 Million | State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	15
Aggregate Fair Value	$ 112,985
Gross Unrealized Loss	$ 341
Unrealized Loss Less than 5 Million | Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities	41
Aggregate Fair Value	$ 220,826
Gross Unrealized Loss	$ 6,904