Summary of Transactions Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013
Shares
Outstanding, Beginning Balance	2,968,582
Outstanding, Ending Balance	2,816,907
Number of Shares Exercisable, Ending Balance	1,683,640
Weighted-Average Exercise Price
Beginning Balance	$ 27.15
Ending Balance	$ 27.55
Weighted Average Exercise Price Exercisable, Ending Balance	$ 24.63
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	5 years 2 months 23 days
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	4 years 5 months 16 days
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of period	$ 60,039,934
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of period	40,792,908
ORCC
Shares
Outstanding, Beginning Balance
Transaction stock options converted upon acquisition of ORCC	37,468
Outstanding, Ending Balance	37,468
Number of Shares Exercisable, Ending Balance	37,468
Weighted-Average Exercise Price
Beginning Balance
Transaction stock options converted upon acquisition of ORCC	$ 91.92
Ending Balance	$ 91.92
Weighted Average Exercise Price Exercisable, Ending Balance	$ 91.92
Weighted-Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	2 years 3 months 18 days
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	2 years 3 months 18 days
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of period	61,350
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of period	$ 61,350