Variable Interest Entities - Narrative (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Variable Interest Entity [Line Items]
Assets, Level 1 to Level 2 transfers	$ 0	$ 0
Assets, Level 2 to Level 1 transfers	0	0
Liabilities, Level 2 to Level 1 transfers	0	0
Liabilities, Level 1 to Level 2 transfers	0	0
Variable Interest Entities
Variable Interest Entity [Line Items]
Assets, Level 1 to Level 2 transfers	0		$ 0
Assets, Level 2 to Level 1 transfers	0		0
Liabilities, Level 2 to Level 1 transfers	0		0
Liabilities, Level 1 to Level 2 transfers	0		$ 0
Investment funds
Variable Interest Entity [Line Items]
Investment funds	41,000,000
Investment funds \| Variable Interest Entities
Variable Interest Entity [Line Items]
Investment funds		22,000,000
Related Party \| Variable Interest Entities
Variable Interest Entity [Line Items]
Equity securities	176,000,000	240,000,000
Related Party \| MidCap Holdings
Variable Interest Entity [Line Items]
Participation interest liability	0	14,000,000
Investment Owned, Balance, Principal Amount	$ 245,000,000	$ 245,000,000

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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For investments which are quantified by principal amount, the principle balance held at close of period.

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The aggregate amount of the participation liability at the end of the accounting period.

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