Stockholders' Equity Treasury Stock (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Balance at beginning of year	27,236,516	22,455,507	20,315,380
Shares repurchased	5,350,976	5,076,530	2,272,213
Shares issued to fund stock option exercises	(133,951)	(109,560)	(24,024)
Employee stock purchase plan shares issued	(51,658)	(72,707)	(62,866)
Nonvested shares issued	(111,003)	(124,031)	(51,191)
Nonvested shares forfeited	14,198	10,777	5,995
Balance at end of year	32,305,078	27,236,516	22,455,507