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Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Derivative liabilities $ 0 $ 498,000  
Payments to acquire other investments 41,000 103,000 $ 61,000
Restricted cash and cash equivalents 20,300,000 56,000,000  
Not Designated as Hedging Instrument | TBA      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Derivative liabilities   148,000,000  
Proceeds from sale of investments 0 114,400,000  
Payments to acquire other investments 0 262,400,000  
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Derivative in liability position   0  
Derivative liability netted against asset position   133,500  
Collateral for Hedging Instruments [Member]      
Derivative Instruments and Hedging Activities Disclosures [Line Items]      
Restricted cash and cash equivalents $ 9,900,000 $ 6,100,000