Number of Outside Managers Managing Investment Portfolio

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Related Party Transactions (Details)	12 Months Ended
Related Party Transactions (Details)	Dec. 31, 2019 USD ($) manager	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Related Party Transaction [Line Items]
Number of investment portfolio managers \| manager	6
Net expenses from transactions with related parties	$ 3,800,000	$ 4,000,000.0	$ 4,100,000
Due from affiliates	0	0
Accounts payable, related parties	0	0
Accounts receivable, related parties	0	0
Due to Related Parties	2,000,000	2,000,000
Equity Method Investments [Member]
Related Party Transaction [Line Items]
Revenue from related parties	$ 1,000,000.0	$ 1,200,000
Affiliated Entity [Member]
Related Party Transaction [Line Items]
Number of investment portfolio managers \| manager	2
BlackRock Financial Management, Inc. [Member] \| Common Stock [Member] \| Affiliated Entity [Member]
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners	5.00%
Wellington Management Company LLP [Member] \| Common Stock [Member] \| Affiliated Entity [Member]
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners	5.00%