Long-Term Debt, Convertible Note (Details)				3 Months Ended	4 Months Ended	12 Months Ended
	Apr. 18, 2018 USD ($)	Feb. 28, 2018 shares	Jun. 16, 2016 USD ($)	Mar. 31, 2018 USD ($)	Apr. 17, 2018 USD ($)	Dec. 31, 2018 USD ($) d $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Convertible Debt [Abstract]
Equity component of convertible notes allocated to additional paid-in capital								$ 6,825,000
Debt issuance costs						$ 7,243,000	$ 0	6,596,000
Interest expense						21,800,000	22,200,000	15,600,000
Loss on extinguishment of debt				$ (7,200,000)		(7,220,000)	$ 0	$ 0
Class A Common Stock [Member]
Convertible Debt [Abstract]
Shares issued (in shares) | shares		1,535,652
Convertible Note [Member]
Convertible Debt [Abstract]
Original principal amount			$ 210,000,000			$ 210,000,000
Interest rate						4.75%
Maturity date						Jun. 15, 2020
Redemption price, percentage of principal amount redeemed						100.00%
Threshold percentage of stock price trigger						180.00%
Threshold trading days | d						20
Threshold final trading days | d						3
Threshold consecutive trading days | d						30
Principal amount increments that can be repurchased upon a change in control or termination of trading of common stock						$ 1,000
Principal amount increments that can be converted						$ 1,000
Holding period following issue date before notes can be converted						6 months
Initial conversion rate (in shares) | shares						21.5054
Initial conversion price (in dollars per share) | $ / shares						$ 46.50
Proceeds from issuance of notes			210,000,000
Liability component of convertible notes			199,100,000
Equity component of convertible notes allocated to additional paid-in capital			10,900,000
Effective interest rate						7.10%
Debt issuance costs			6,596,000
Additions to deferred financing cost			6,300,000
Adjustments to additional paid-in capital for debt issuance costs			$ 300,000
Interest expense						$ 3,500,000
Contractual interest						3,000,000
Amortization of debt issuance costs and debt discount						$ 500,000
Payment for principal and accrued interest	$ 213,400,000
Accrued interest					$ 3,400,000