Equity - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Series B
Stockholders Equity Note [Line Items]
Interest rate determination	2.05%		2.05%
Preferred return percentage earned on unreturned preferred capital	1.20%		1.20%
Accumulated undistributed preferred returns	$ 19.8	$ 18.9
Series D
Stockholders Equity Note [Line Items]
Ownership interest	1.00%
Preferred return percentage earned on unreturned preferred capital	7.00%
Accumulated undistributed preferred returns	$ 46.4	$ 40.3
Limited Partner
Stockholders Equity Note [Line Items]
Ownership interest	78.38%
General Partner
Stockholders Equity Note [Line Items]
Ownership interest	20.62%