Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted Investments	$ 41,800,000		$ 41,800,000
Goodwill and intangible impairment	0
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial and nonfinancial liabilities, fair value disclosure	0		0
Significant Other Observable Remaining Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	2,200,000,000		2,252,900,000
Privately-held debt and redeemable preferred stock securities | Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on investments	0	$ 0
Junos Pulse
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Note receivable	$ 61,200,000		$ 61,200,000