Shareholders' Equity (Equity Component) (Details) Tangible_Equity_Unit in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 Tangible_Equity_Unit shares	Jul. 02, 2016 trade_day $ / shares $ / per_TEU shares	Jun. 30, 2018 trade_day Tangible_Equity_Unit shares
Debt Instrument [Line Items]
Fair value price per TEU (in dollars per tangible equity unit) | $ / per_TEU		100
Equity Component
Debt Instrument [Line Items]
Fair value price per TEU (in dollars per tangible equity unit) | $ / per_TEU		76.19
Threshold consecutive trading days | trade_day		20	20
Threshold trading days | trade_day		23
Settlement of tangible equity units, units settled | Tangible_Equity_Unit	473
Settlement of tangible equity units, shares of common stock issued (in shares)	939,000
Tangible equity units prepaid stock purchase contracts, outstanding units | Tangible_Equity_Unit	677		677
Equity Component | Common Class A
Debt Instrument [Line Items]
Threshold stock price (in dollars per share) | $ / shares		$ 50.40
Debt instrument convertible reference price (in dollars per share) | $ / shares		$ 42.00
Equity Component | Minimum | Common Class A
Debt Instrument [Line Items]
Convertible, number of equity instruments per contract (in shares)	1.9841	1.9841	1.9841
Equity Component | Maximum | Common Class A
Debt Instrument [Line Items]
Convertible, number of equity instruments per contract (in shares)		2.3810	2.3810