Stock-Based Compensation Plans (Summary of Stock Option Activity) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding at Beginning of Year (in shares)	11,837	14,252	17,692
Shares, Granted (in shares)	1,613	1,295	3,227
Shares, Exercised (in shares)	(4,278)	(3,476)	(6,291)
Shares, Forfeited (in shares)	(228)	(234)	(376)
Shares, Outstanding at End of Year (in shares)	8,944	11,837	14,252
Shares, Exercisable at End of Year (in shares)	4,101	5,297	4,855
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Wtd. Avg. Ex. Price, Outstanding at Beginning of Year (in dollars per share)	$ 16	$ 15	$ 14
Wtd. Avg. Ex. Price, Granted (in dollars per share)	22	20	17
Wtd. Avg. Ex. Price, Exercised (in dollars per share)	15	13	12
Wtd. Avg. Ex. Price, Forfeited (in dollars per share)	18	16	15
Wtd. Avg. Ex. Price, Outstanding at End of Year (in dollars per share)	18	16	15
Wtd. Avg. Ex. Price, Exercisable at End of Year (in dollars per share)	$ 16	$ 15	$ 14
Option expiration period, years	2 years 9 months 18 days	2 years 8 months 12 days	3 years
Wtd. Avg. Remaining Contract Life, Exercisable at End of Year	1 year 8 months 12 days	2 years	2 years 3 months 18 days
Aggregate Intrinsic Value, Exercised	$ 38,097	$ 25,156	$ 31,790
Aggregate Intrinsic Value, Outstanding at End of Year	24,881	58,202	67,763
Aggregate Intrinsic Value, Exercisable at End of Year	$ 16,162	$ 32,152	$ 30,021
Non-Employee Director Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding at Beginning of Year (in shares)	406	381	373
Shares, Granted (in shares)	57	56	56
Shares, Exercised (in shares)	(101)	(31)	(36)
Shares, Forfeited (in shares)	0	0	(12)
Shares, Outstanding at End of Year (in shares)	362	406	381
Shares, Exercisable at End of Year (in shares)	354	406	381
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Wtd. Avg. Ex. Price, Outstanding at Beginning of Year (in dollars per share)	$ 15	$ 14	$ 13
Wtd. Avg. Ex. Price, Granted (in dollars per share)	24	19	16
Wtd. Avg. Ex. Price, Exercised (in dollars per share)	13	14	10
Wtd. Avg. Ex. Price, Forfeited (in dollars per share)	0	0	16
Wtd. Avg. Ex. Price, Outstanding at End of Year (in dollars per share)	16	15	14
Wtd. Avg. Ex. Price, Exercisable at End of Year (in dollars per share)	$ 16	$ 15	$ 14
Option expiration period, years	6 years	6 years 1 month 6 days	6 years 4 months 24 days
Wtd. Avg. Remaining Contract Life, Exercisable at End of Year	6 years	6 years 1 month 6 days	6 years 4 months 24 days
Aggregate Intrinsic Value, Exercised
Aggregate Intrinsic Value, Outstanding at End of Year	1,582	2,565	2,180
Aggregate Intrinsic Value, Exercisable at End of Year	$ 1,582	$ 2,565	$ 2,180