Shareholders' Equity - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Common shares authorized (shares)	100,000,000	100,000,000
Authorized preferred stock (shares)	50,000,000	50,000,000
Authorization common shares for the issuance under incentive compensation plans (shares)	2,100,000
Number of shares available for grant (shares)	600,000
Dividends declared	$ 316,900	$ 315,000	$ 119,900
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity attributable to parent	1,594,795	1,798,702	1,958,354	$ 2,157,944
Total authorizations which remain available for use (shares)	109,600
Loss related to unrecognized changes in defined benefit plan liabilities		1,000	$ 1,000
Impairment losses in accumulated other comprehensive income	$ 500	$ 300
Effective federal tax rate	35.00%
Retained Earnings, Unappropriated
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity attributable to parent	$ 239,100