Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Investments	$ 3,403,822,000us-gaap_Investments	$ 3,158,312,000us-gaap_Investments
Total return swap	0	1,650,000
Credit facility amount	150,000,000
Investments [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Investments	3,403,822,000	3,158,312,000
Interest Rate Swap [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Interest rate swap agreements, liabilities	4,025,000	0
Secured Debt [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instrument, fair value	140,000,000	140,000,000
Unsecured Debt [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instrument, fair value	150,000,000	50,000,000
Equity Contract [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Equity contracts	194,000	140,000
Revolving Credit Facility [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Credit facility amount	$ 150,000,000

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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Amount borrowed under the credit facility as of the balance sheet date.

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