Fair Value Measurements - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fluctuation in discount rate (basis points)	100.00%
Assets held for sale	$ 26,664	$ 18,741
Investment in receivable portfolios, carrying amount	3,137,893	2,890,613	$ 2,382,809	$ 2,440,669
Deferred court costs, net	95,918	79,963
Convertible and exchangeable senior notes, carrying value	656,000	483,500
Unamortized debt discount	36,361	32,720
Senior notes, carrying value	325,000	326,029
United States
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease of the fair value	45,300
Europe
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease of the fair value	67,400
Other geographies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease of the fair value	5,500
Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible and exchangeable senior notes, carrying value	619,639	450,780
Unamortized debt discount	36,400	32,700
Cabot senior secured notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unamortized debt discount	1,500	1,900
Level 2 | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value estimate of convertible senior notes incorporates quoted market prices	553,744	520,944
Cabot senior secured notes | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes, carrying value	1,111,399
Cabot senior secured notes | Secured Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes, carrying value	1,109,922	1,214,558
Cabot senior secured notes | Level 2 | Secured Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes, estimated fair value	1,036,905	1,258,935
Fair Value | Certain Loans Acquired in Transfer Not Accounted for as Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in receivable portfolios, estimated fair value	$ 3,525,861	$ 3,415,325