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Senior Notes - Principal Terms of Senior Notes Outstanding (Details) - Senior Notes	12 Months Ended
	Dec. 31, 2018 USD ($)
2024 Senior Notes
Debt Instrument
Par value (in millions)	$ 400,000,000	[1]
Stated coupon	4.25%	[1]
Redemption price percentage	100.00%
2024 Senior Notes | Treasury Rate
Debt Instrument
Applicable basis spread over redemption price percentage	0.25%
2025 Senior Notes
Debt Instrument
Par value (in millions)	$ 350,000,000	[1]
Stated coupon	3.50%	[1]
Redemption price percentage	100.00%
2025 Senior Notes | Treasury Rate
Debt Instrument
Applicable basis spread over redemption price percentage	0.25%
[1] The senior notes may be redeemed at any time, in whole or in part, at a make-whole redemption price plus accrued and unpaid interest.

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