Stock-Based Compensation - Non-cash compensation expense and capitalized non-cash compensation (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Non-cash compensation expense and capitalized non-cash compensation
Nonvested restricted stock and restricted stock units (RSUs)	6
Stock-based compensation expense	$ 48	$ 47
Outperform Stock Appreciation Rights
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	0	1
Restricted Stock Units
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	28	21
Performance Restricted Stock Units
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	9	14
401 (K) Match Expense
Non-cash compensation expense and capitalized non-cash compensation
Stock-based compensation expense	$ 11	$ 11