Senior Notes - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Jan. 10, 2013 6.000% Senior Notes due 2043 [Member]	Mar. 31, 2013 6.000% Senior Notes due 2043 [Member]
Proforma Debt Instrument [Line Items]
Proceeds from issuance of senior notes	$ 247,813,000		$ 247,800,000
Total Debt Obligations	995,032,000	744,842,000	250,000,000	250,000,000
Underwriting fees			$ 2,200,000