Stock-Based Compensation Plans and Employee Benefits - Performance Incentive Plans (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) shares ownership_class $ / shares	Mar. 31, 2018 USD ($) shares	Dec. 31, 2018 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation \| $	$ 4,249	$ 9,091
Performance Incentive Plan
Accrued expenses \| $	$ 71,305		$ 95,149
iPIP Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	6 years
Performance Incentive Plan
iPIP distributions to plan participants \| $	$ 7,400
iPIP distributions, cash component \| $	$ 3,800
iPIP distributions, equity component (in shares)	389,545
iPIP distributions, equity component, fair value \| $	$ 3,600
iPIP distributions, equity component, fair value (in dollars per share) \| $ / shares	$ 9.21
iPIP distributions, shares issued net of tax withholdings (in shares)	209,118
Accrued expenses \| $	$ 33,000		$ 37,500
iPIP Plans \| Vesting at end of second year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights percentage	40.00%
iPIP Plans \| Vesting in each of years three through six
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rights percentage	15.00%
2013-2014 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	85.77		85.77
Granted (in shares)	0.00		0.50
Forfeited (in shares)	(0.00)		(1.30)
Points at end of period (in shares)	85.77	86.57	86.57
2015-2016 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	78.59		79.41
Granted (in shares)	0.00		0.00
Forfeited (in shares)	(0.82)		(4.75)
Points at end of period (in shares)	79.41	84.16	84.16
2017-2018 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	81.40		82.43
Granted (in shares)	0.00		49.33
Forfeited (in shares)	(1.03)		(7.87)
Points at end of period (in shares)	82.43	40.97	40.97
2019-2020 Performance Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of ownership classes of consolidated subsidiary \| ownership_class	2
Ownership percentage of class A units (percent)	100.00%
Equity-based compensation \| $	$ 400
Settlement or distribution on units and awards, cash (percent)	50.00%
Settlement or distribution on units and awards, common stock (percent)	50.00%
2013-2018 Performance Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Settlement or distribution on units and awards, cash (percent)	50.00%
Settlement or distribution on units and awards, common stock (percent)	50.00%

Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include property accounts payable, legal liabilities, loan costs, general and administrative liabilities, but exclude interest.

No definition available.