Schedule I Summary Of Investments (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	$ 4,475,362
Summary of Investments, Other than Investments in Related Parties, Fair Value	4,465,024
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	4,462,430
US Treasury and Government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	78,931
Summary of Investments, Other than Investments in Related Parties, Fair Value	75,474
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	75,474
US States and Political Subdivisions Debt Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	66,124	[1]
Summary of Investments, Other than Investments in Related Parties, Fair Value	67,171	[1]
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	67,171	[1]
Other Corporate Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	1,463,720
Summary of Investments, Other than Investments in Related Parties, Fair Value	1,455,628
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	1,455,628
RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	358,262
Summary of Investments, Other than Investments in Related Parties, Fair Value	350,628
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	350,628
CMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	429,057
Summary of Investments, Other than Investments in Related Parties, Fair Value	428,289
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	428,289
Other ABS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	433,603
Summary of Investments, Other than Investments in Related Parties, Fair Value	434,728
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	434,728
Foreign government securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	24,771
Summary of Investments, Other than Investments in Related Parties, Fair Value	24,594
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	24,594
Other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	2,000
Summary of Investments, Other than Investments in Related Parties, Fair Value	2,000
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	2,000
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	2,856,468
Summary of Investments, Other than Investments in Related Parties, Fair Value	2,838,512
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	2,838,512
Trading Assets, Excluding Debt and Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	874,764	[2]
Summary of Investments, Other than Investments in Related Parties, Fair Value	879,862	[2]
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	879,862	[2]
Common Stocks, by Industry [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	830
Summary of Investments, Other than Investments in Related Parties, Fair Value	830
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	830
Nonredeemable Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	500
Summary of Investments, Other than Investments in Related Parties, Fair Value	500
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	500
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	1,330
Summary of Investments, Other than Investments in Related Parties, Fair Value	1,330
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	1,330
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	741,605
Summary of Investments, Other than Investments in Related Parties, Fair Value	741,531
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	741,531
Other Long-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Summary of Investments, Other than Investments in Related Parties, Cost	1,195
Summary of Investments, Other than Investments in Related Parties, Fair Value	3,789
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	$ 1,195

[1] Available for sale.

[2] Includes foreign government and agency securities.