Investment Information - Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 lots	Dec. 31, 2013 lots	Dec. 31, 2012
Segment Reporting Information [Line Items]
Investable assets	$ 15,773,209,000	$ 14,049,525,000	$ 13,045,134,000
OTTI unrealized losses at current fair value	900,000	6,000,000
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots)	1,900	2,080
Total number of positions (lots)	4,790	4,400
Largest single loss	2,900,000	3,500,000
OTTI Unrealized Losses	3,646,000	12,942,000
Securities lending, qualitative disclosures:
Fair value of fixed maturities and short-term investments pledged under securities lending agreements	50,800,000	105,100,000
Amortized cost of fixed maturities and short-term investments pledged under securities lending agreements	52,100,000	105,900,000
Approximate fair value of sub-prime securities backing the securities lending program	5,800,000	6,300,000
Equity Method Investment, Financial Statement, Reported Amounts [Abstract]
Equity in net income (loss) of investment funds accounted for using the equity method	19,883,000	35,701,000	73,510,000
Equity method investments, time lag for reporting - low end of range	1 month
Equity method investments, time lag for reporting - high end of range	3 months
Investments accounted for using the equity method	349,014,000	244,339,000
ACGL
Segment Reporting Information [Line Items]
Investable assets	14,610,000,000
Watford Re
Segment Reporting Information [Line Items]
Investable assets	$ 1,160,000,000