Debt - Narrative (Detail) - USD ($)					1 Months Ended	3 Months Ended				6 Months Ended
	May. 15, 2015	Mar. 20, 2014	Mar. 13, 2012	Apr. 09, 2010	Mar. 31, 2012	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Amount of difference between fair value and book value						$ 311,500,000				$ 311,500,000		$ 259,500,000
Weighted average interest rate on borrowings										2.00%	2.00%
Repayment of Debt										$ 270,800,000	$ 811,000,000
Loss on debt extinguishment						0		$ 0		0	(77,100,000)
Interest paid, net of interest received						$ 1,200,000		$ 2,200,000		$ 2,900,000	4,700,000
4.95% Senior Notes Due April 1, 2014
Debt Instrument [Line Items]
Repayment of Debt					$ 341,800,000				$ 258,200,000
Interest rate				4.95%	4.95%	4.95%				4.95%
Maturity date				Apr. 01, 2014	Apr. 01, 2014					Apr. 01, 2014
5.50% Senior Notes Due May 15, 2015
Debt Instrument [Line Items]
Repayment of Debt	$ 200,000,000
Interest rate						5.50%				5.50%
Maturity date										May 15, 2015
Senior notes												200,000,000
6.00% Senior Notes Due due April 1, 2024
Debt Instrument [Line Items]
Interest rate						6.00%				6.00%
Maturity date							Apr. 01, 2024			Apr. 01, 2024
Senior notes		$ 400,000,000				$ 400,000,000				$ 400,000,000		400,000,000
8.25% Senior Notes Due March 15, 2019
Debt Instrument [Line Items]
Repayment of Debt		$ 211,100,000									211,100,000
Interest rate		8.25%	8.25%			8.25%				8.25%
Maturity date		Mar. 15, 2019	Mar. 15, 2019							Mar. 15, 2019
Senior notes			$ 450,000,000			$ 238,900,000				$ 238,900,000		238,900,000
7.25% Senior Notes Due May 15, 2018
Debt Instrument [Line Items]
Repayment of Debt		$ 100,000,000
Interest rate		7.25%				7.25%				7.25%
Maturity date		May 15, 2018								May 15, 2018
Senior notes						$ 250,000,000				$ 250,000,000		250,000,000
7.625% Senior Notes Due June 15, 2020
Debt Instrument [Line Items]
Repayment of Debt		$ 50,000,000
Interest rate		7.625%				7.625%				7.625%
Maturity date		Jun. 15, 2020								Jun. 15, 2020
Senior notes						$ 350,000,000				$ 350,000,000		$ 350,000,000
Debt Instruments Extinguished Three
Debt Instrument [Line Items]
Loss on debt extinguishment											77,100,000
Gain due to termination of related interest rate swaps											$ 2,800,000
Credit Agreements
Debt Instrument [Line Items]
Allowable annual dividend payment under credit agreement						225,000,000				225,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Total amount of revolving credit agreement						1,500,000,000				$ 1,500,000,000
Line of credit facility, expiration date										Sep. 09, 2019
Line of credit facility amendment date										Sep. 09, 2014
Scenario, Previously Reported | Revolving Credit Facility
Debt Instrument [Line Items]
Total amount of revolving credit agreement						$ 1,150,000,000				$ 1,150,000,000
Line of credit facility, expiration date										Oct. 15, 2017