Investments - Schedule of Investments Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	$ 3,099,723	$ 3,127,792
Other Investments, Original or amortized cost	10,438	4,522
Total investments, Original or amortized cost	3,110,161	3,132,314
Available-for-sale securities, Gross unrealized gains	149,734	100,648
Other investments, Gross unrealized gains	623	570
Investments, Gross unrealized gains	150,357	101,218
Available-for-sale securities, Gross unrealized losses	(23,781)	(66,373)
Other investments, Gross unrealized losses	0	0
Investments, Gross unrealized losses	(23,781)	(66,373)
Fair value	3,225,676	3,162,067
Other investments	11,061	5,092
Total investments	3,236,737	3,167,159
U.S. treasury bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	11,956	16,622
Available-for-sale securities, Gross unrealized gains	505	587
Available-for-sale securities, Gross unrealized losses	0	0
Fair value	12,461	17,209
U.S. agency bonds – mortgage-backed [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	1,215,829	1,292,032
Available-for-sale securities, Gross unrealized gains	15,959	11,727
Available-for-sale securities, Gross unrealized losses	(19,202)	(41,104)
Fair value	1,212,586	1,262,655
U.S. agency bonds – other [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	7,209	7,207
Available-for-sale securities, Gross unrealized gains	965	901
Available-for-sale securities, Gross unrealized losses	0	0
Fair value	8,174	8,108
Non-U.S. government bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	65,755	70,377
Available-for-sale securities, Gross unrealized gains	3,818	3,547
Available-for-sale securities, Gross unrealized losses	(15)	(712)
Fair value	69,558	73,212
Other mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	43,925	33,676
Available-for-sale securities, Gross unrealized gains	1,623	0
Available-for-sale securities, Gross unrealized losses	0	(232)
Fair value	45,548	33,444
Corporate bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	1,692,907	1,546,578
Available-for-sale securities, Gross unrealized gains	123,494	82,952
Available-for-sale securities, Gross unrealized losses	(4,564)	(22,830)
Fair value	1,811,837	1,606,700
Municipal bonds – auction rate [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost		99,170
Available-for-sale securities, Gross unrealized gains		0
Available-for-sale securities, Gross unrealized losses		0
Fair value		99,170
Municipal bonds – other [Member]
Schedule of Investments [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	62,142	62,130
Available-for-sale securities, Gross unrealized gains	3,370	934
Available-for-sale securities, Gross unrealized losses	0	(1,495)
Fair value	$ 65,512	$ 61,569