Business Combinations - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Jul. 01, 2018	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019
Business Acquisition [Line Items]
Drawdown from existing revolving credit facility						$ 8,800	$ 79,150
Adjustment to redemption value			$ 16,604
Redeemable noncontrolling interest in subsidiaries		$ 199,492			$ 182,513	199,492
Hermes
Business Acquisition [Line Items]
Redeemable noncontrolling interest in subsidiaries		187,700				187,700
Hermes | Restricted Stock | Put Option
Business Acquisition [Line Items]
Period after vesting	9 months
Hermes | Restricted Stock | Minimum
Business Acquisition [Line Items]
Vesting period	3 years
Hermes | Restricted Stock | Maximum
Business Acquisition [Line Items]
Vesting period	5 years
Hermes
Business Acquisition [Line Items]
Payments to acquire business					$ 344,300			$ 343,500
Cash received from acquisition			800
Drawdown from existing revolving credit facility	$ 18,000
Fair value of Redeemable noncontrolling interest	33.00%
Increase in Intangible Assets			43,800
Increase in Other Long-Term Assets			5,000
Increase in Long-Term Deferred Tax Liability			8,200
Decrease in Goodwill			$ 41,800
Revenue		49,300		$ 49,700		145,900
Net income		$ 3,000		$ 5,700		$ 5,500
Hermes
Business Acquisition [Line Items]
Remaining interest contributed to employee benefit trust	10.50%
Hermes | BTPS
Business Acquisition [Line Items]
Ownership interest retained	29.50%