Summary of Dividends Declared (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dividends Payable [Line Items]
Total Amount	$ 130,423	$ 76,550	$ 79,076
Q1 2012
Dividends Payable [Line Items]
Declaration Date	Feb. 09, 2012
Dividend Per Share	$ 0.09
Record Date	Mar. 12, 2012
Total Amount	12,204
Payment Date	Mar. 30, 2012
Q2 2012
Dividends Payable [Line Items]
Declaration Date	Apr. 25, 2012
Dividend Per Share	$ 0.09
Record Date	May 30, 2012
Total Amount	12,205
Payment Date	Jun. 19, 2012
Q3 2012
Dividends Payable [Line Items]
Declaration Date	Jul. 25, 2012
Dividend Per Share	$ 0.13
Record Date	Aug. 28, 2012
Total Amount	18,061
Payment Date	Sep. 18, 2012
Q1 2011
Dividends Payable [Line Items]
Declaration Date		Feb. 09, 2011
Dividend Per Share		$ 0.14
Record Date		Mar. 11, 2011
Total Amount		19,352
Payment Date		Mar. 31, 2011
Q2 2011
Dividends Payable [Line Items]
Declaration Date		Apr. 27, 2011
Dividend Per Share		$ 0.14
Record Date		May 27, 2011
Total Amount		19,232
Payment Date		Jun. 17, 2011
Q3 2011
Dividends Payable [Line Items]
Declaration Date		Jul. 26, 2011
Dividend Per Share		$ 0.14
Record Date		Aug. 26, 2011
Total Amount		19,148
Payment Date		Sep. 16, 2011
Q4 2011
Dividends Payable [Line Items]
Declaration Date		Oct. 26, 2011
Dividend Per Share		$ 0.14
Record Date		Nov. 18, 2011
Total Amount		18,818
Payment Date		Dec. 09, 2011
Q1 2010
Dividends Payable [Line Items]
Declaration Date			Feb. 10, 2010
Dividend Per Share			$ 0.14
Record Date			Mar. 11, 2010
Total Amount			20,220
Payment Date			Mar. 31, 2010
Q2 2010
Dividends Payable [Line Items]
Declaration Date			Apr. 27, 2010
Dividend Per Share			$ 0.14
Record Date			May 27, 2010
Total Amount			19,902
Payment Date			Jun. 17, 2010
Q3 2010
Dividends Payable [Line Items]
Declaration Date			Jul. 26, 2010
Dividend Per Share			$ 0.14
Record Date			Aug. 26, 2010
Total Amount			19,703
Payment Date			Sep. 16, 2010
Q4 2010
Dividends Payable [Line Items]
Declaration Date			Oct. 25, 2010
Dividend Per Share			$ 0.14
Record Date			Nov. 18, 2010
Total Amount			19,251
Payment Date			Dec. 09, 2010
October Twenty Twelve | Q4 2012
Dividends Payable [Line Items]
Declaration Date	Oct. 24, 2012
Dividend Per Share	$ 0.13
Record Date	Nov. 16, 2012
Total Amount	17,658
Payment Date	Dec. 07, 2012
December Twenty Twelve | Q4 2012
Dividends Payable [Line Items]
Declaration Date	Dec. 07, 2012
Dividend Per Share	$ 0.52
Record Date	Dec. 17, 2012
Total Amount	$ 70,295
Payment Date	Dec. 28, 2012