Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Investment securities
Fair value unfunded commitments	$ 5.3
Private equity funds
Investment securities
Other long-term investments	125.2
Number of Investments Held	38
Hedge funds
Investment securities
Other long-term investments	114.6
Number of Investments Held	16
Total hedge and private equity funds included in other long-term investments
Investment securities
Other long-term investments	239.8		254.2
Fair value unfunded commitments	119.4	[1]	81.5
Other Long-term Investments Largest Single Investment	16.0		27.4
Hedge funds
Investment securities
Other long-term investments	114.6		125.3
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long/short equity
Investment securities
Other long-term investments	60.3		48.8
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long/short credit & distressed
Investment securities
Other long-term investments	22.7		32.3
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long diversified strategies
Investment securities
Other long-term investments	1.7		16.9
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long/short equity REIT
Investment securities
Other long-term investments	16.0		14.5
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long/short equity activist
Investment securities
Other long-term investments	13.6		12.3
Fair value unfunded commitments	0	[1]	0
Hedge funds | Long bank loan
Investment securities
Other long-term investments	0.3		0.5
Fair value unfunded commitments	0	[1]	0
Private equity funds
Investment securities
Other long-term investments	125.2		128.9
Fair value unfunded commitments	119.4	[1]	81.5
Private equity funds | Multi-sector
Investment securities
Other long-term investments	23.3		26.9
Fair value unfunded commitments	5.4	[1]	8.2
Private equity funds | Energy infrastructure & services
Investment securities
Other long-term investments	36.3		28.0
Fair value unfunded commitments	15.6	[1]	9.9
Private equity funds | Distressed residential real estate
Investment securities
Other long-term investments	15.8		27.4
Fair value unfunded commitments	0	[1]	0
Private equity funds | Real estate
Investment securities
Other long-term investments	11.6		9.5
Fair value unfunded commitments	3.3	[1]	3.3
Private equity funds | Private equity secondaries
Investment securities
Other long-term investments	10.5		11.3
Fair value unfunded commitments	3.1	[1]	4.0
Private equity funds | International multi-sector, Europe
Investment securities
Other long-term investments	5.1		7.8
Fair value unfunded commitments	5.0	[1]	4.7
Private equity funds | Manufacturing/Industrial
Investment securities
Other long-term investments	9.9		6.2
Fair value unfunded commitments	15.1	[1]	0
Private equity funds | Healthcare
Investment securities
Other long-term investments	4.3		2.3
Fair value unfunded commitments	5.4	[1]	7.0
Private equity funds | International multi-sector, Asia
Investment securities
Other long-term investments	0.4		3.6
Fair value unfunded commitments	2.7	[1]	2.6
Private equity funds | Insurance
Investment securities
Other long-term investments	3.0		3.5
Fair value unfunded commitments	41.3	[1]	41.3
Private equity funds | Aerospace/Defense/Government
Investment securities
Other long-term investments	2.8		0
Fair value unfunded commitments	22.2	[1]	0
Private equity funds | Venture capital
Investment securities
Other long-term investments	2.2		2.4
Fair value unfunded commitments	$ 0.3	[1]	$ 0.5

[1] White Mountains also has unfunded commitments to a tax advantaged federal affordable housing development fund of $5.3 at December 31, 2012.