Accounting Policies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) company loan shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($)	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Accounting Policies [Line Items]
Number of bank loans on nonaccrual status | loan	0
Allowance for doubtful accounts	$ 5,700	$ 3,900
Bad debt expense	2,500	1,500	$ 1,000
Allowance for doubtful accounts receivables written off	812	313	408
Fee income	9,100	13,900	17,000
Assets	(5,024,405)	(3,136,776)	$ (2,756,695)
Liabilities	(4,245,824)	(2,427,535)
Investment in variable interest entities	$ 31,200	$ 29,800
Common share equivalents excluded from calculations of diluted earnings per share | shares	9,735	180,329	174,380
Right-of-use assets	$ 17,200
Lease liabilities	18,232
Bank loan participations held-for-investment	$ 260,864	$ 260,972
Minimum
Accounting Policies [Line Items]
Intangible assets, useful life (in years)	7 years
Property and equipment, estimated useful lives (in years)	3 years
Maximum
Accounting Policies [Line Items]
Intangible assets, useful life (in years)	27 years 6 months
Property and equipment, estimated useful lives (in years)	10 years
Receivables greater than 90 days past due
Accounting Policies [Line Items]
Premiums receivable, past due	$ 3,300	$ 4,500
United States
Accounting Policies [Line Items]
Number of insurance companies | company	5
ASU 2018-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU							$ 0
ASU 2016-02
Accounting Policies [Line Items]
Assets					$ 22,600
Liabilities					30,900
Cumulative effect of adoption of ASU					8,280
Right-of-use assets					17,200
Lease liabilities					$ 17,800
AOCI | ASU 2018-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU						$ 711
Forecast | ASU 2016-13
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU				$ (7,800)
Bank loan participations held-for-investment				$ (8,400)