Employee Benefit Plans (ESPP) (Details) shares in Millions		3 Months Ended
	Nov. 06, 2017 period	Mar. 31, 2018 $ / shares shares	Mar. 31, 2017 $ / shares shares
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
ESPP offering period duration (in months)	24 months
ESPP, number of offering period | period	4
ESPP. purchase period (in months)	6 months
ESPP, purchase price of common stock (in percent)	85.00%
ESPP, rolling years	2 years
Employee stock purchase plan 2008
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Stock issued during period, shares, employee stock purchase plans (in shares) | shares		1.3	1.5
Average price of common stock, per share (in dollars per share) | $ / shares		$ 22.23	$ 19.21