Stock-Based Compensation Plans (Summary of Stock Option Activity) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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)	14,252	17,692	17,829
Shares, Granted (in shares)	1,295	3,227	2,966
Shares, Exercised (in shares)	(3,476)	(6,291)	(2,344)
Shares, Forfeited (in shares)	(234)	(376)	(759)
Shares, Outstanding at End of Year (in shares)	11,837	14,252	17,692
Shares, Exercisable at End of Year (in shares)	5,297	4,855	6,858
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)	20	17	17
Wtd. Avg. Ex. Price, Exercised (in dollars per share)	13	12	12
Wtd. Avg. Ex. Price, Forfeited (in dollars per share)	16	15	14
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)	$ 15	$ 14	$ 13
Option expiration period, years	2 years 8 months 12 days	3 years	3 years
Wtd. Avg. Remaining Contract Life, Exercisable at End of Year	2 years	2 years 3 months 18 days	2 years 1 month 6 days
Aggregate Intrinsic Value, Exercised	$ 25,156	$ 31,790	$ 11,596
Aggregate Intrinsic Value, Outstanding at End of Year	58,202	67,763	45,842
Aggregate Intrinsic Value, Exercisable at End of Year	$ 32,152	$ 30,021	$ 23,917
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)	381	373	381
Shares, Granted (in shares)	56	56	49
Shares, Exercised (in shares)	(31)	(36)	(57)
Shares, Forfeited (in shares)	0	(12)	0
Shares, Outstanding at End of Year (in shares)	406	381	373
Shares, Exercisable at End of Year (in shares)	406	381	373
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)	$ 14	$ 13	$ 12
Wtd. Avg. Ex. Price, Granted (in dollars per share)	19	16	17
Wtd. Avg. Ex. Price, Exercised (in dollars per share)	14	10	11
Wtd. Avg. Ex. Price, Forfeited (in dollars per share)	0	16	0
Wtd. Avg. Ex. Price, Outstanding at End of Year (in dollars per share)	15	14	13
Wtd. Avg. Ex. Price, Exercisable at End of Year (in dollars per share)	$ 15	$ 14	$ 13
Option expiration period, years	6 years 1 month 6 days	6 years 4 months 24 days	6 years 4 months 24 days
Wtd. Avg. Remaining Contract Life, Exercisable at End of Year	6 years 1 month 6 days	6 years 4 months 24 days	6 years 4 months 24 days
Aggregate Intrinsic Value, Exercised
Aggregate Intrinsic Value, Outstanding at End of Year	2,565	2,180	1,069
Aggregate Intrinsic Value, Exercisable at End of Year	$ 2,565	$ 2,180	$ 1,069