Fair Value (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Financial Assets and Liabilities, Measured at Fair Value Recurring Basis [Table Text Block]

The following table presents a summary of fair value measurements by level and carrying values for items measured at fair value on a recurring basis in the Consolidated Balance Sheets:

(in millions)	Quoted Prices in Active Markets (Level 1)			Other Observable Inputs (Level 2)			Unobservable Inputs (Level 3)			Total Fair and Carrying Value
December 31, 2017
Cash and cash equivalents	$	11,718		$	263		$	—		$	11,981
Debt securities - available-for-sale:
U.S. government and agency obligations	2,428			216			—			2,644
State and municipal obligations	—			7,660			—			7,660
Corporate obligations	65			12,989			140			13,194
U.S. agency mortgage-backed securities	—			3,911			—			3,911
Non-U.S. agency mortgage-backed securities	—			1,015			—			1,015
Total debt securities - available-for-sale	2,493			25,791			140			28,424
Equity securities	1,784			14			194			1,992
Assets under management	1,117			1,984			—			3,101
Total assets at fair value	$	17,112		$	28,052		$	334		$	45,498
Percentage of total assets at fair value	38		%	61		%	1		%	100		%
December 31, 2016
Cash and cash equivalents	$	10,386		$	44		$	—		$	10,430
Debt securities - available-for-sale:
U.S. government and agency obligations	2,017			247			—			2,264
State and municipal obligations	—			7,059			—			7,059
Corporate obligations	21			10,804			102			10,927
U.S. agency mortgage-backed securities	—			2,927			—			2,927
Non-U.S. agency mortgage-backed securities	—			1,002			—			1,002
Total debt securities - available-for-sale	2,038			22,039			102			24,179
Equity securities	1,591			13			437			2,041
Assets under management	1,064			2,041			—			3,105
Total assets at fair value	$	15,079		$	24,137		$	539		$	39,755
Percentage of total assets at fair value	38		%	61		%	1		%	100		%

Fair Value Measurements, Nonrecurring [Table Text Block]

The following table presents a summary of fair value measurements by level and carrying values for certain financial instruments not measured at fair value on a recurring basis in the Consolidated Balance Sheets:

(in millions)	Quoted Prices in Active Markets (Level 1)		Other Observable Inputs (Level 2)		Unobservable Inputs (Level 3)		Total Fair Value		Total Carrying Value
December 31, 2017
Debt securities - held-to-maturity:
U.S. government and agency obligations	$	251	$	3	$	—	$	254	$	254
State and municipal obligations	—		—		2		2		2
Corporate obligations	16		1		263		280		280
Total debt securities - held-to-maturity	$	267	$	4	$	265	$	536	$	536
Long-term debt and other financing obligations	$	—	$	34,504	$	—	$	34,504	$	31,542
December 31, 2016
Debt securities - held-to-maturity:
U.S. government and agency obligations	$	251	$	—	$	—	$	251	$	250
State and municipal obligations	—		—		5		5		5
Corporate obligations	20		8		210		238		238
Total debt securities - held-to-maturity	$	271	$	8	$	215	$	494	$	493
Long-term debt and other financing obligations	$	—	$	31,295	$	—	$	31,295	$	29,337