Debt - Additional Information Domain (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended												12 Months Ended						12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Northwind Holdings LLC [Member]	Dec. 31, 2010 Northwind Holdings LLC [Member]	Dec. 31, 2009 Northwind Holdings LLC [Member]	Dec. 31, 2007 Northwind Holdings LLC [Member]	Dec. 31, 2011 Tailwind Holdings LLC [Member]	Dec. 31, 2006 Tailwind Holdings LLC [Member]	Dec. 31, 2010 Notes 5.625 Percent due 2020 callable at or above par [Member]	Dec. 31, 2009 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2010 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2010 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2009 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2009 Notes at 6.75 Percent due 2028 callable at or above par [Member]	Dec. 31, 2011 Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]	Dec. 31, 1998 Junior Subordinated Debt Securities 7.405 Percent due 2038 [Member]	Dec. 31, 1998 Debt Securities 7.405 Percent due 2038 issued to Provident Financing Trust [Member]	Dec. 31, 2009 Notes @ 5.859 - Short term [Member]	Dec. 31, 2011 Notes 7.625 Percent due 2011 callable at or above par [Member]	Dec. 31, 2010 Receive Fixed/Pay Variable Swap [Member]
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 296.9
Long-term Debt, Maturities, Repayments of Principal in Year Five	350.0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,915.1
Proceeds from Issuance of Senior Long-term Debt							800.0		130.0
Debt Instrument, Basis Spread on Variable Rate				0.78%				0.35%
Debt service coverage account (DSCA)				12.7				18.2
Principal payments on senior secured non-recourse variable rate notes				74.4	58.3	48.0		10.0
Repayments of Long-term Debt	84.4	78.3	59.8																132.2	225.1
Debt Instrument, Interest Rate, Stated Percentage												7.125%	7.08%	7.19%	6.75%		7.405%	7.405%	5.859%	7.625%
Proceeds from Unsecured Notes Payable										400.0	350.0
Repayments of Medium-term Notes													10.0
Debt Instrument, Decrease, Repayments														1.2	0.6
Notional Amount of Fair Value Hedge Instruments																					350.0
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	8.8	(14.4)
Junior Subordinated Notes																	300.0
Liquidation value per capital security																$ 1,000
Repayments of Other Debt			58.3
Interest Paid	$ 145.4	$ 140.7	$ 122.0