Employee Benefit Plans - Employee Stock Purchase Plan (Details) shares in Millions		3 Months Ended		9 Months Ended
	Nov. 06, 2017 period	Sep. 30, 2019 $ / shares shares	Sep. 30, 2018 $ / shares shares	Sep. 30, 2019 $ / shares shares	Sep. 30, 2018 $ / shares shares
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESPP offering period duration	24 months
ESPP, number of purchase period | period	4
ESPP. purchase period	6 months
ESPP, purchase price of common stock	85.00%
Employee stock purchase plan 2008
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Shares purchased (in shares) | shares		1.2	1.2	2.4	2.5
Average exercise price per share (in dollars per share) | $ / shares		$ 22.03	$ 22.39	$ 22.04	$ 22.31