Leases​ (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Leases [Abstract]
Lease cost	$ 15	$ 14	$ 45	$ 43
Right-of-use asset obtained in exchange for operating lease liability			13	$ 38
Operating Lease Obligation
2021	11		11		$ 43
2022	38		38		34
2023	30		30		26
2024	22		22		20
2025	17		17		16
Thereafter	40		40		41
Total lease payments	158		158		180
Less: imputed interest	25		25		33
Present value of lease liabilities	$ 133		$ 133		$ 147
Weighted-average remaining lease term	6 years 2 months 12 days		6 years 2 months 12 days		6 years 7 months 6 days
Weighted-average discount rate	5.80%		5.80%		5.90%