Share-Based Compensation Arrangements (Nonvested Performance Shares Outstanding) (Details) - Performance Based Nonvested Shares [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share awards, shares vesting	117,465
2017 Grant [Domain]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share awards granted, grant date fair value | $ / shares	$ 51.85
Performance share awards, shares vesting percentages	140.00%
Performance share awards, shares vesting	36,470
2016 Grant [Domain]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share awards granted, grant date fair value | $ / shares	$ 36.11
Performance share awards, shares vesting percentages	133.00%
Performance share awards, shares vesting	45,220
2015 Grant [Domain]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance share awards granted, grant date fair value | $ / shares	$ 38.02
Performance share awards, shares vesting percentages	135.00%
Performance share awards, shares vesting	35,775