Subsequent Events (Details) - 2019 Tender Offer - Subsequent Event $ / shares in Units, $ in Millions	Feb. 28, 2019 USD ($) $ / shares shares
Subsequent Events
Shares authorized for repurchase per tender offer | shares	150,000
Shares authorized for repurchase per tender offer, as a percentage of oustanding common stock	3.80%
Price per share of common stock per tender offer | $ / shares	$ 159.63
Aggregate purchase price of shares per tender offer | $	$ 23.9