Incentive Plans - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Unit Based Compensation Arrangement [Line Items]
Market value of common units under the incentive plan	Average closing price over the last 20 trading days prior to the vesting date
Vesting period of Grants in years	4 years
Partnership's historical forfeiture rate	4.00%
Partnership's historical distribution rate	7.10%
Expenses related to Incentive Plan to be reimbursed to general partner	$ 5.1	$ 1.5
Payments made in connection with Long-Term Incentive Plan	6.6	6.5
Unaccrued cost associated with outstanding grants and related DERs	$ 15.3
Minimum [Member]
Unit Based Compensation Arrangement [Line Items]
Risk free interest rate	0.21%
Volatility rate	29.64%
Maximum [Member]
Unit Based Compensation Arrangement [Line Items]
Risk free interest rate	0.54%
Volatility rate	34.71%