Equity and NonControllling Interests - Schedule of Redeemable Common Units (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Equity And Noncontrolling Interests [Line Items]
Redeemable common units, beginning of period	$ 7,159
Comprehensive income	55,963	35,674
Conversion of redeemable common units for shares		(591)
Adjustment for ownership interest of redeemable common units	(6)	(382)
Stock-based compensation	1,764	1,423
Distributions to common unitholders	(31,665)	(25,331)
Adjustment to redemption value of redeemable common units	134	(355)
Redeemable common units, end of period	7,093
Redeemable Common Units [Member]
Equity And Noncontrolling Interests [Line Items]
Redeemable common units, beginning of period	7,159	6,840
Comprehensive income	141	95
Conversion of redeemable common units for shares		(591)
Adjustment for ownership interest of redeemable common units	6	382
Stock-based compensation	4	4
Distributions to common unitholders	(83)	(69)
Adjustment to redemption value of redeemable common units	(134)	355
Redeemable common units, end of period	$ 7,093	$ 7,016