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, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Northwind Holdings LLC [Member]	Dec. 31, 2011 Northwind Holdings LLC [Member]	Dec. 31, 2010 Northwind Holdings LLC [Member]	Dec. 31, 2007 Northwind Holdings LLC [Member]	Dec. 31, 2012 Tailwind Holdings LLC [Member]	Dec. 31, 2006 Tailwind Holdings LLC [Member]	Dec. 31, 2012 Notes @ 5.75% due 2042, callable at or above par	Dec. 31, 2009 Notes 7.125 Percent due 2016 callable at or above par [Member]	Dec. 31, 2009 Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2012 Notes @ 6.85% due 2015, callable at or above par	Dec. 31, 2012 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 1998 Junior Subordinated Debt Securities 7 Point 405 Percent due 2038 [Member]	Dec. 31, 1998 Debt Securities 7.405 Percent due 2038 issued to Provident Financing Trust [Member]	Dec. 31, 2010 Notes 7.625 Percent due 2011 callable at or above par [Member] Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2012 Notes 7.625 Percent due 2011 callable at or above par [Member] Medium-term Notes 7.0 Percent to 7.2 Percent due 2023 to 2028 noncallable [Member]	Dec. 31, 2012 Receive Fixed/Pay Variable Swap [Member]	Dec. 31, 2011 Receive Fixed/Pay Variable Swap [Member]	Dec. 31, 2012 Repayment of Debt [Member] Tailwind Holdings LLC [Member]
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 296.9
Long-term Debt, Maturities, Repayments of Principal in Year Four	350.0
Long-term Debt, Maturities, Repayments of Principal after Year Five	2,095.1
Debt Instrument, Basis Spread on Variable Rate				0.78%				0.35%
Debt service coverage account (DSCA)				29.3				21.5
Debt Instrument, Periodic Payment, Principal				60.0	74.4	58.3		10.0
Repayments of Long-term Debt	70.0	84.4	78.3																		62.5
Proceeds from Issuance of Senior Long-term Debt							800.0		130.0
Proceeds from Unsecured Notes Payable										250.0	400.0
Debt Instrument, Decrease, Repayments												10.0
Notional Amount of Fair Value Hedge Instruments																			600.0	350.0
Junior Subordinated Notes															300.0
Liquidation value per capital security														$ 1,000
Debt Instrument, Interest Rate, Stated Percentage												7.08%	6.85%		7.405%	7.405%		7.625%
Repayments of Medium-term Notes																	225.1
Interest Paid	$ 139.6	$ 145.4	$ 140.7