Summary of Significant Accounting Policies Accounts Payable (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accounts Payable	$ 125	$ 127
Accrued Salaries, Current	62	48
Accrued Exchange Fee Rebate, Current	44	34
Taxes Payable, Current	15	7
Operating Lease, Liability, Current	8	8
Other Accrued Liabilities, Current	13	18
Accounts payable and accrued liabilities	267	242
Unpaid capital expenditures	$ 16	$ 15	$ 22