Debt Obligations - Senior Unsecured Notes (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Dec. 19, 2013	Dec. 18, 2012	Sep. 17, 2012
5.00% Senior Notes due 2022
Debt Instrument [Line Items]
Long-term Debt					$ 600
Debt stated interest rate	5.00%	5.00%			5.00%
Debt percentage of principle amount notes were sold at					100.00%
4.625% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt				$ 500
Debt stated interest rate	4.625%	4.625%		4.625%
Debt percentage of principle amount notes were sold at				100.00%
5.875% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt			$ 400
Debt stated interest rate	5.875%	5.875%	5.875%
Debt percentage of principle amount notes were sold at			100.00%
6.375% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term Debt			$ 700
Debt stated interest rate	6.375%	6.375%	6.375%
Debt percentage of principle amount notes were sold at			100.00%