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Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) quote / Investment	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2018 USD ($) quote / Investment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2.2					2.2		2.0
Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 85.0
Silverton 2015, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 70.0
Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 125.0
Silverton 2016, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 100.0
Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued			$ 130.0
Silverton 2017, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued			$ 105.0
Fair Value, Measurements, Recurring | Level 3 | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt								$ 115.6
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled		$ 1.4					$ 115.9
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled	$ 14.7					$ 78.4

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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The fair value of notes issued in noncash investing and financing activities.

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