Discontinued Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues									$ 197	$ 11,369	$ 17,661
Gain on sale									16,369	5,431	10,566
Income from discontinued operations	$ 4,709	$ 8,287	$ 2	$ 1,686	$ 6,444	$ 2,638	$ 123	$ 2,990	$ 14,684	$ 12,195	$ 19,983
Income from discontinued operations - diluted per share/unit									$ 0.11	$ 0.11	$ 0.20