Shareholders' Equity - Common Stock and Debt Repurchase Authorizations (Details) - USD ($)	Dec. 31, 2017	Jul. 21, 2016
Debt Securities
Class of Stock [Line Items]
Debt repurchase program, amount authorized		$ 150,000,000
Principal amount of debt securities repurchased	$ 100,000,000
Common Stock
Class of Stock [Line Items]
Authorized repurchase of securities, with debt reduction target		$ 100,000,000