Notes and Loan Payable and Amounts Due Under Repurchase Agreements Notes and Loan Payable and Amounts Due Under Repurchase Agreements (2027 Notes Narrative) (Details) (Details) - USD ($) $ in Thousands			3 Months Ended				12 Months Ended
	Jul. 17, 2017	Jun. 16, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Loss on extinguishment of debt			$ 0	$ 18,389	$ 428	$ 0	$ 18,817	$ 0	$ 0
June 2027 Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount		$ 500,000	500,000				500,000	0
Interest rate		5.00%
Unamortized debt issue costs		$ (300)	$ (335)				$ (335)	$ 0
Deferred financing costs		5,800
Term Loan [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount		$ 100,000
July 2021 Notes [Member]
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 400,000
Extinguishment of debt, cash paid	$ 413,300
Debt instrument, redemption price, percentage	3.313%
Loss on extinguishment of debt	$ 18,400