Variable Interest Entities (Details) $ in Billions	6 Months Ended
	Jun. 30, 2018 USD ($) LimitedPartnerships
Variable Interest Entity [Line Items]
Methodology for determining whether the Company is primary beneficiary	When the Company becomes involved with a variable interest entity, as well as when there is a change in the Company’s involvement with an entity, the Company must determine if it is the primary beneficiary and must consolidate the entity. The Company would be considered the primary beneficiary if it has the power to direct the entity’s most significant economic activities or has the right to receive benefits or obligation to absorb losses that could be significant to the entity. The Company evaluates the following criteria: the structure and purpose of the entity; the risks and rewards created by and shared through the entity; and the Company’s ability to direct its activities, receive its benefits and absorb its losses relative to the other parties involved with the entity including its sponsors, equity holders, guarantors, creditors and servicers. The Company determined it was not a primary beneficiary in any material variable interest entities as of June 30, 2018 or December 31, 2017. The Company’s involvement in variable interest entities where it is not the primary beneficiary is described below.
Securities limited partnerships and real estate limited partnerships [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss related to arrangements with variable interest entity	$ 2.7
Number of limited partnerships defined as variable interest entities | LimitedPartnerships	125
Carrying amount of assets	$ 1.4
Commitments to contribute additional cash, amount	$ 1.3
Securities limited partnerships and real estate limited partnerships [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Non-controlling interest percentage	10.00%
Other asset-backed and corporate securities [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss related to arrangements with variable interest entity	$ 0.6
Carrying amount of assets	$ 0.6
Non-controlling interest percentage
Real estate joint ventures [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss related to arrangements with variable interest entity
Carrying amount of assets
Independent physician associations [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss related to arrangements with variable interest entity
Carrying amount of assets
India Joint Venture [Member]
Variable Interest Entity [Line Items]
Maximum exposure to loss related to arrangements with variable interest entity
Carrying amount of assets