Schedule of Debt Outstanding (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Notes issued with a discount	8,455,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving Credit Facility maturity date	Dec. 31, 2016
Weighted average rate	1.70%	1.00%
Senior Notes interest payable dates	interest payable periodically at variable interest rates
Senior Notes 6.25% Due 2013
Debt Instrument [Line Items]
Interest rate stated percentage of Senior Notes	6.25%
Note maturity date	Mar. 15, 2013
Senior Notes interest payable dates	interest payable semi-annually in March and September at 6.25%.
Senior Notes 6.75% Due 2014
Debt Instrument [Line Items]
Interest rate stated percentage of Senior Notes	6.75%
Note maturity date	Jan. 31, 2014
Senior Notes interest payable dates	interest payable semi-annually in January and July at 6.75%.
Senior Notes 7.75% Due 2017
Debt Instrument [Line Items]
Interest rate stated percentage of Senior Notes	7.75%
Note maturity date	Jun. 01, 2017
Senior Notes interest payable dates	interest payable semi-annually in June and December at 7.75%.
Notes issued with a discount	13,400,000
Notes, unamortized	8,500,000	10,000,000