Stock-Based Compensation Plans and Employee Benefits (Performance Incentive Plans) (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) shares $ / shares	Dec. 31, 2017 USD ($)
Performance Incentive Plan
Preferred return (as a percent)	9.00%
Reduction in amount distributable (percent)	4.30%
Accrued expenses | $	$ 97,007	$ 101,035
iPIP Plans
Performance Incentive Plan
iPIP distributions to plan participants | $	15,600
iPIP distributions, cash component | $	$ 7,800
iPIP distributions, equity component (in shares)	685,624
iPIP distributions, equity component, fair value | $	$ 7,800
iPIP distributions, equity component, fair value (in dollars per share) | $ / shares	$ 11.37
iPIP distributions, shares issued net of tax withholdings (in shares)	374,055
Accrued expenses | $	$ 37,500	$ 38,100
2013-2014 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	85.77
Granted (in shares)	0.50
Forfeited (in shares)	(1.30)
Points at end of period (in shares)	86.57
2015-2016 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	79.41
Granted (in shares)	0.00
Forfeited (in shares)	(4.75)
Points at end of period (in shares)	84.16
2017-2018 Performance Incentive Plan
Performance Incentive Plan
Points at beginning of period (in shares)	82.43
Granted (in shares)	49.33
Forfeited (in shares)	(7.87)
Points at end of period (in shares)	40.97