Stock-Based Compensation Employee stock purchase plan (Details)	12 Months Ended
	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / shares shares	Dec. 31, 2014 $ / shares shares
Employee Stock Ownership Plan (ESOP) disclosures [Line Items]
Number of employees participating	1,952	1,750	1,684
Shares purchased during the year	148,631	125,779	115,029
Weighted-average per share purchase price paid | $ / shares	$ 28.34	$ 26.22	$ 23.07
Cumulative shares purchased since 1982	9,025,665	8,877,034	8,751,255