Fair Value Measurement (Details) (USD $)	Jun. 30, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets:
Short-term investments	$ 1,070,401,000	$ 1,031,578,000
Other invested assets	252,082,000	283,032,000
Credit derivative assets	603,867,000	592,898,000
FG VIEs' assets, at fair value	3,492,204,000	3,657,481,000	762,300,000
Liabilities:
Credit derivative liabilities	2,788,218,000	2,465,520,000
FG VIEs' liabilities with recourse, at fair value	2,755,113,000	2,926,988,000
FG VIEs' liabilities without recourse, at fair value	1,282,463,000	1,337,214,000
Fair value measurement on recurring basis
Assets:
Total fixed maturity securities	9,864,200,000	9,415,300,000
Short-term investments	1,070,400,000	1,031,600,000
Other invested assets	37,900,000	33,300,000
Credit derivative assets	603,900,000	592,900,000
FG VIEs' assets, at fair value	3,492,200,000	3,657,500,000
Other assets	49,300,000	44,400,000
Total assets carried at fair value	15,117,900,000	14,775,000,000
Liabilities:
Credit derivative liabilities	2,788,200,000	2,465,500,000
FG VIEs' liabilities with recourse, at fair value	2,755,100,000	2,927,000,000
FG VIEs' liabilities without recourse, at fair value	1,282,500,000	1,337,200,000
Other liabilities		100,000
Total liabilities carried at fair value	6,825,800,000	6,729,800,000
Fair value measurement on recurring basis | Level 1
Assets:
Short-term investments	129,600,000	253,400,000
Other invested assets	400,000	200,000
Other assets	29,500,000	25,700,000
Total assets carried at fair value	159,500,000	279,300,000
Fair value measurement on recurring basis | Level 2
Assets:
Total fixed maturity securities	9,502,300,000	9,092,700,000
Short-term investments	940,800,000	778,200,000
Other invested assets	25,500,000	21,400,000
Other assets	19,800,000	18,700,000
Total assets carried at fair value	10,488,400,000	9,911,000,000
Liabilities:
Other liabilities		100,000
Total liabilities carried at fair value		100,000
Fair value measurement on recurring basis | Level 2 | U.S. government and agencies
Assets:
Total fixed maturity securities	975,300,000	1,048,200,000
Fair value measurement on recurring basis | Level 2 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,184,200,000	4,959,900,000
Fair value measurement on recurring basis | Level 2 | Corporate securities
Assets:
Total fixed maturity securities	1,065,500,000	992,500,000
Fair value measurement on recurring basis | Level 2 | RMBS
Assets:
Total fixed maturity securities	1,129,500,000	1,071,700,000
Fair value measurement on recurring basis | Level 2 | CMBS
Assets:
Total fixed maturity securities	516,100,000	379,100,000
Fair value measurement on recurring basis | Level 2 | Asset-backed securities
Assets:
Total fixed maturity securities	287,000,000	292,700,000
Fair value measurement on recurring basis | Level 2 | Foreign government securities
Assets:
Total fixed maturity securities	344,700,000	348,600,000
Fair value measurement on recurring basis | Level 3
Assets:
Total fixed maturity securities	361,900,000	322,600,000
Other invested assets	12,000,000	11,700,000
Credit derivative assets	603,900,000	592,900,000
FG VIEs' assets, at fair value	3,492,200,000	3,657,500,000
Total assets carried at fair value	4,470,000,000	4,584,700,000
Liabilities:
Credit derivative liabilities	2,788,200,000	2,465,500,000
FG VIEs' liabilities with recourse, at fair value	2,755,100,000	2,927,000,000
FG VIEs' liabilities without recourse, at fair value	1,282,500,000	1,337,200,000
Total liabilities carried at fair value	6,825,800,000	6,729,700,000
Fair value measurement on recurring basis | Level 3 | RMBS
Assets:
Total fixed maturity securities	88,000,000	112,400,000
Fair value measurement on recurring basis | Level 3 | Asset-backed securities
Assets:
Total fixed maturity securities	273,900,000	210,200,000
Fair value measurement on recurring basis | U.S. government and agencies
Assets:
Total fixed maturity securities	975,300,000	1,048,200,000
Fair value measurement on recurring basis | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,184,200,000	4,959,900,000
Fair value measurement on recurring basis | Corporate securities
Assets:
Total fixed maturity securities	1,065,500,000	992,500,000
Fair value measurement on recurring basis | RMBS
Assets:
Total fixed maturity securities	1,217,500,000	1,184,100,000
Fair value measurement on recurring basis | CMBS
Assets:
Total fixed maturity securities	516,100,000	379,100,000
Fair value measurement on recurring basis | Asset-backed securities
Assets:
Total fixed maturity securities	560,900,000	502,900,000
Fair value measurement on recurring basis | Foreign government securities
Assets:
Total fixed maturity securities	344,700,000	348,600,000
Fair value measurement on non-recurring basis | Level 3
Liabilities:
Mortgage loans recorded at fair value	$ 10,000,000	$ 9,400,000