Debt - Components of Debt (Parenthetical) (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 30, 2017	Dec. 31, 2016	Dec. 31, 2014	Jun. 30, 2014	Aug. 31, 2013
6.750% Senior Notes Due in 2020 [Member]
Debt Instrument [Line Items]
Interest rate on notes	6.75%	6.75%		6.75%		6.75%
Debt instrument maturity year	2020	2020	2020		2020
Interest rate on notes	6.75%	6.75%		6.75%		6.75%
Debt instrument maturity year	2020	2020	2020		2020
10.000% Senior Notes Due in 2021 [Member]
Debt Instrument [Line Items]
Interest rate on notes				10.00%				10.00%
Debt instrument maturity year				2021	2021
Interest rate on notes				10.00%				10.00%
Debt instrument maturity year				2021	2021
5.375% Senior Notes Due in 2022 [Member]
Debt Instrument [Line Items]
Interest rate on notes	5.375%	5.375%		5.375%			5.375%
Debt instrument maturity year	2022	2022	2022		2022
Interest rate on notes	5.375%	5.375%		5.375%			5.375%
Debt instrument maturity year	2022	2022	2022		2022
5.500% Senior Notes Due in 2024 [Member]
Debt Instrument [Line Items]
Interest rate on notes				5.50%
Debt instrument maturity year				2024	2024
Interest rate on notes				5.50%
Debt instrument maturity year				2024	2024
5.500% Senior Notes Due in 2025 [Member]
Debt Instrument [Line Items]
Interest rate on notes				5.50%
Debt instrument maturity year				2025	2025
Interest rate on notes				5.50%
Debt instrument maturity year				2025	2025