Derivative Instruments	12 Months Ended
Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments Interest Rate Derivatives—We are exposed to risks arising from our business operations, economic conditions and financial markets. To manage these risks, we primarily use interest rate derivatives to hedge our debt and our cash flows. The interest rate derivatives include interest rate caps and interest rate floors, which are subject to master netting settlement arrangements. All derivatives are recorded at fair value. During the year ended December 31, 2017, we entered into interest rate caps with notional amounts totaling $844.2 million and strike rates ranging from 3.00% to 11.61%. These interest rate caps had effective dates from January 2017 to December 2017, maturity dates from March 2018 to September 2019, and a total cost of $375,000. These instruments were not designated as cash flow hedges. We also entered into interest rate floors with notional amounts of $3.9 billion and strike rates ranging from 1.00% to 1.50%. These interest rate floors had effective dates from September 2017 to December 2017 and maturity dates from March 2019 to June 2019 and a total cost of $140,000. During the year ended December 31, 2016, we entered into interest rate caps with notional amounts totaling $224.5 million and strike rates ranging from 5.43% to 5.78%. These interest rate caps had effective dates from March 2016 to December 2016, maturity dates from March 2017 to December 2017 and total costs of $13,000. These instruments were not designated as cash flow hedges. As of December 31, 2017, we held interest rate caps with notional amounts totaling $887.7 million and strike rates ranging from 2.00% to 11.61%. These instruments cap the interest rates on our mortgage loans with an aggregate principal balances of $818.1 million and maturity dates from March 2018 to August 2022. These instruments have maturity dates ranging from January 2018 to September 2019. As of December 31, 2017, we held interest rate floors with notional amounts totaling $6.9 billion and strike rates ranging from -0.25% to 1.50%. These instruments have maturity dates ranging from March 2019 to July 2020. Credit Default Swap Derivatives—We use credit default swaps, tied to the CMBX index, to hedge financial and capital market risk. A credit default swap is a derivative contract that functions like an insurance policy against the credit risk of an entity or obligation. The seller of protection assumes the credit risk of the reference obligation from the buyer (us) of protection in exchange for annual premium payments. If a default or a loss, as defined in the credit default swap agreements, occurs on the underlying bonds, then the buyer of protection is protected against those losses. The only liability for us, the buyer, is the annual premium and any change in value of the underlying CMBX index (if the trade is terminated prior to maturity). For all CMBX trades completed to date, we were the buyer of protection. Credit default swaps are subject to master-netting settlement arrangements and credit support annexes. As of December 31, 2017, we held a credit default swap with a notional amount of $50.0 million, an effective date of August 2017 and an expected maturity date of October 2026. Assuming the underlying bonds pay off at par over their remaining average life, our estimated total exposure for these trades was approximately $2.4 million as of December 31, 2017. Cash collateral is posted by us as well as our counterparties. We offset the fair value of the derivative and the obligation/right to return/reclaim cash collateral. The change in market value of credit default swaps is settled net through posting cash collateral or reclaiming cash collateral between us and our counterparties when such change in market value is over $250,000. Options on Futures Contracts—During the year ended December 31, 2016, we purchased an option on Eurodollar futures for a total cost of $124,000 and a maturity date of June 2017. During the year ended December 31, 2017, we made no such purchases.

Derivative Instruments

Interest Rate Derivatives—We are exposed to risks arising from our business operations, economic conditions and financial markets. To manage these risks, we primarily use interest rate derivatives to hedge our debt and our cash flows. The interest rate derivatives include interest rate caps and interest rate floors, which are subject to master netting settlement arrangements. All derivatives are recorded at fair value.

During the year ended December 31, 2017, we entered into interest rate caps with notional amounts totaling $844.2 million and strike rates ranging from 3.00% to 11.61%. These interest rate caps had effective dates from January 2017 to December 2017, maturity dates from March 2018 to September 2019, and a total cost of $375,000. These instruments were not designated as cash flow hedges. We also entered into interest rate floors with notional amounts of $3.9 billion and strike rates ranging from 1.00% to 1.50%. These interest rate floors had effective dates from September 2017 to December 2017 and maturity dates from March 2019 to June 2019 and a total cost of $140,000.

During the year ended December 31, 2016, we entered into interest rate caps with notional amounts totaling $224.5 million and strike rates ranging from 5.43% to 5.78%. These interest rate caps had effective dates from March 2016 to December 2016, maturity dates from March 2017 to December 2017 and total costs of $13,000. These instruments were not designated as cash flow hedges.

As of December 31, 2017, we held interest rate caps with notional amounts totaling $887.7 million and strike rates ranging from 2.00% to 11.61%. These instruments cap the interest rates on our mortgage loans with an aggregate principal balances of $818.1 million and maturity dates from March 2018 to August 2022. These instruments have maturity dates ranging from January 2018 to September 2019.

As of December 31, 2017, we held interest rate floors with notional amounts totaling $6.9 billion and strike rates ranging from -0.25% to 1.50%. These instruments have maturity dates ranging from March 2019 to July 2020.

Credit Default Swap Derivatives—We use credit default swaps, tied to the CMBX index, to hedge financial and capital market risk. A credit default swap is a derivative contract that functions like an insurance policy against the credit risk of an entity or obligation. The seller of protection assumes the credit risk of the reference obligation from the buyer (us) of protection in exchange for annual premium payments. If a default or a loss, as defined in the credit default swap agreements, occurs on the underlying bonds, then the buyer of protection is protected against those losses. The only liability for us, the buyer, is the annual premium and any change in value of the underlying CMBX index (if the trade is terminated prior to maturity). For all CMBX trades completed to date, we were the buyer of protection. Credit default swaps are subject to master-netting settlement arrangements and credit support annexes. As of December 31, 2017, we held a credit default swap with a notional amount of $50.0 million, an effective date of August 2017 and an expected maturity date of October 2026. Assuming the underlying bonds pay off at par over their remaining average life, our estimated total exposure for these trades was approximately $2.4 million as of December 31, 2017. Cash collateral is posted by us as well as our counterparties. We offset the fair value of the derivative and the obligation/right to return/reclaim cash collateral. The change in market value of credit default swaps is settled net through posting cash collateral or reclaiming cash collateral between us and our counterparties when such change in market value is over $250,000.

Options on Futures Contracts—During the year ended December 31, 2016, we purchased an option on Eurodollar futures for a total cost of $124,000 and a maturity date of June 2017. During the year ended December 31, 2017, we made no such purchases.