Capital Stock - Dividends (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Cash Dividends [Line Items]
Per Share Amount	$ 0.42	$ 0.42	$ 0.84	$ 0.84
Year 2013 [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 1.395
Total			$ 4,662,000
Year 2014 [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 2.16
Total			$ 22,643,000
Year 2015 [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 1.92
Total			$ 38,748,000
Year 2016 [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 1.68
Total			$ 41,388,000
Year 2017 [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 0.98
Total			$ 36,977,000
Cumulative Total [Member]
Cash Dividends [Line Items]
Per Share Amount			$ 8.135
Total			$ 144,418,000
Subsequent Event Dividend Declaration [Member]
Cash Dividends [Line Items]
Declaration Date			7/12/2017
Payment Date			8/10/2017
Per Share Amount			$ 0.14