Fair Value Measurements - Assets at Fair Value Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	$ 1,806	$ 1,798
Other securities	76	86
U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	12	11
Obligations of states, municipalities, and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	86	92
Corporate debt
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	1,184	1,098
RMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	218	217
CMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	63	57
CDO/ABS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	78	85
Level 1
Investments, Debt and Equity Securities [Abstract]
Total investment securities	42	45
Level 2
Investments, Debt and Equity Securities [Abstract]
Total investment securities	1,833	1,835
Level 3
Investments, Debt and Equity Securities [Abstract]
Total investment securities	7	4
Fair Value, Measurements, Recurring
Assets
Cash equivalents in mutual funds	1,534	775
Cash equivalents in securities		68
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	1,806	1,798
Other securities	76	86
Total investment securities	1,882	1,884
Restricted cash equivalents in mutual funds	487	403
Total	3,903	3,130
Fair Value, Measurements, Recurring | Total bonds
Investments, Debt and Equity Securities [Abstract]
Other securities	38	40
Fair Value, Measurements, Recurring | U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	12	11
Fair Value, Measurements, Recurring | Obligations of states, municipalities, and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	86	92
Fair Value, Measurements, Recurring | Commercial paper
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	24	91
Fair Value, Measurements, Recurring | Non-U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	141	147
Other securities	1	1
Fair Value, Measurements, Recurring | Corporate debt
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	1,184	1,098
Other securities	20	24
Fair Value, Measurements, Recurring | RMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	218	217
Other securities	1	1
Fair Value, Measurements, Recurring | CMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	63	57
Fair Value, Measurements, Recurring | CDO/ABS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	78	85
Other securities	16	14
Fair Value, Measurements, Recurring | Preferred stock
Investments, Debt and Equity Securities [Abstract]
Other securities	14	19
Fair Value, Measurements, Recurring | Common stock
Investments, Debt and Equity Securities [Abstract]
Other securities	24	26
Fair Value, Measurements, Recurring | Other long-term investments
Investments, Debt and Equity Securities [Abstract]
Other securities		1
Fair Value, Measurements, Recurring | Level 1
Assets
Cash equivalents in mutual funds	1,534	775
Cash equivalents in securities		0
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	5	5
Other securities	37	40
Total investment securities	42	45
Restricted cash equivalents in mutual funds	487	403
Total	2,063	1,223
Fair Value, Measurements, Recurring | Level 1 | Total bonds
Investments, Debt and Equity Securities [Abstract]
Other securities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Obligations of states, municipalities, and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Non-U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	5	5
Other securities	0	0
Fair Value, Measurements, Recurring | Level 1 | RMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	0
Fair Value, Measurements, Recurring | Level 1 | CMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | CDO/ABS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Preferred stock
Investments, Debt and Equity Securities [Abstract]
Other securities	14	14
Fair Value, Measurements, Recurring | Level 1 | Common stock
Investments, Debt and Equity Securities [Abstract]
Other securities	23	26
Fair Value, Measurements, Recurring | Level 1 | Other long-term investments
Investments, Debt and Equity Securities [Abstract]
Other securities		0
Fair Value, Measurements, Recurring | Level 2
Assets
Cash equivalents in mutual funds	0	0
Cash equivalents in securities		68
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	1,795	1,793
Other securities	38	42
Total investment securities	1,833	1,835
Restricted cash equivalents in mutual funds	0	0
Total	1,833	1,903
Fair Value, Measurements, Recurring | Level 2 | Total bonds
Investments, Debt and Equity Securities [Abstract]
Other securities	38	37
Fair Value, Measurements, Recurring | Level 2 | U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	12	11
Fair Value, Measurements, Recurring | Level 2 | Obligations of states, municipalities, and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	86	92
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	24	91
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	141	147
Other securities	1	1
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	1,179	1,093
Other securities	20	23
Fair Value, Measurements, Recurring | Level 2 | RMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	212	217
Other securities	1	1
Fair Value, Measurements, Recurring | Level 2 | CMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	63	57
Fair Value, Measurements, Recurring | Level 2 | CDO/ABS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	78	85
Other securities	16	12
Fair Value, Measurements, Recurring | Level 2 | Preferred stock
Investments, Debt and Equity Securities [Abstract]
Other securities	0	5
Fair Value, Measurements, Recurring | Level 2 | Common stock
Investments, Debt and Equity Securities [Abstract]
Other securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Other long-term investments
Investments, Debt and Equity Securities [Abstract]
Other securities		0
Fair Value, Measurements, Recurring | Level 3
Assets
Cash equivalents in mutual funds	0	0
Cash equivalents in securities		0
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	6	0
Other securities	1	4
Total investment securities	7	4
Restricted cash equivalents in mutual funds	0	0
Total	7	4
Fair Value, Measurements, Recurring | Level 3 | Total bonds
Investments, Debt and Equity Securities [Abstract]
Other securities	0	3
Fair Value, Measurements, Recurring | Level 3 | U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Obligations of states, municipalities, and political subdivisions
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Non-U.S. government and government sponsored entities
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	1
Fair Value, Measurements, Recurring | Level 3 | RMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	6	0
Other securities	0	0
Fair Value, Measurements, Recurring | Level 3 | CMBS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | CDO/ABS
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities	0	0
Other securities	0	2
Fair Value, Measurements, Recurring | Level 3 | Preferred stock
Investments, Debt and Equity Securities [Abstract]
Other securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Common stock
Investments, Debt and Equity Securities [Abstract]
Other securities	$ 1	0
Fair Value, Measurements, Recurring | Level 3 | Other long-term investments
Investments, Debt and Equity Securities [Abstract]
Other securities		$ 1