Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available for Sale Amortized Cost
Due in one year or less	$ 710,510
Due after one year through five years	849,257
Due after five years through ten years	679,681
Due after ten years	479,093
Total	3,429,932	3,827,496
Available for Sale Securities Fair Value
Due in one year or less	723,337
Due after one year through five years	928,836
Due after five years through ten years	739,249
Due after ten years	513,586
Total	3,638,268	4,203,450
Residential Mortgage Backed Securities [Member]
Available for Sale Amortized Cost
Debt Maturities, without single maturity date	161,450
Available for Sale Securities Fair Value
Debt Maturities, without Single Maturity Date	167,359
Commercial Mortgage Backed Securities [Member]
Available for Sale Amortized Cost
Debt Maturities, without single maturity date	(372,540)
Available for Sale Securities Fair Value
Debt Maturities, without Single Maturity Date	(381,262)
Asset-backed Securities [Member]
Available for Sale Amortized Cost
Debt Maturities, without single maturity date	177,401
Available for Sale Securities Fair Value
Debt Maturities, without Single Maturity Date	$ 184,639