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Senior Notes and Unsecured Revolving Credit Facility - Schedule of Senior Notes (Phantoms) (Detail)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
May 31, 2016
4.375% Senior notes due 2019 [Member]        
Debt Instrument [Line Items]        
Maturity date of senior note Jun. 15, 2019      
5.875% Senior notes due 2024 [Member]        
Debt Instrument [Line Items]        
Maturity date of senior note Jun. 15, 2024      
Senior Notes [Member] | 4.375% Senior notes due 2019 [Member]        
Debt Instrument [Line Items]        
Interest rate on senior note (percent)   4.375% 4.375%  
Maturity date of senior note   Jun. 15, 2019 Jun. 15, 2019  
Senior Notes [Member] | 4.875% Senior Notes due 2021 [Member]        
Debt Instrument [Line Items]        
Interest rate on senior note (percent)   4.875% 4.875% 4.875%
Maturity date of senior note   Jul. 01, 2021 Jul. 01, 2021  
Senior Notes [Member] | 5.875% Senior notes due 2024 [Member]        
Debt Instrument [Line Items]        
Interest rate on senior note (percent)   5.875% 5.875%  
Maturity date of senior note   Jun. 15, 2024 Jun. 15, 2024