Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
		Feb. 28, 2018	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Shares [Member]
Number of Shares [Rollforward]
As of January 1			127,898	127,580	119,540	119,635
Granted	[1]			50,729	57,189	43,583
Unearned Grant Award				(40,801)	(42,126)	(36,670)
Forfeited				(9,610)	(7,023)	(7,008)
As of December 31				127,898	127,580	119,540
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 58.23	$ 52.56	$ 52.72	$ 48.26
Granted	[1]			62.90	52.43	58.95
Unearned Grant Award				46.27	52.70	45.41
Forfeited				58.29	53.45	53.49
As of December 31				$ 58.23	$ 52.56	$ 52.72
Restricted Stock Units [Member]
Number of Shares [Rollforward]
As of January 1			55,248	54,728	57,694	53,888
Granted	[2]			21,241	20,351	26,702
Awarded				(17,281)	(19,661)	(19,464)
Forfeited				(3,440)	(3,656)	(3,432)
As of December 31				55,248	54,728	57,694
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 56.18	$ 51.79	$ 49.86	$ 44.47
Granted	[2]			62.20	50.25	54.81
Awarded				49.72	44.33	41.44
Forfeited				56.00	52.87	51.52
As of December 31				$ 56.18	$ 51.79	$ 49.86
Subsequent Event [Member] | Performance Shares [Member]
Number of Shares [Rollforward]
Granted			35,894
Awarded		(58,293)
Performance Period (Years)			3 years
Granted, Grant Date Fair Value			$ 2.9
Awarded, Grant Date Fair Value		$ 3.5
Subsequent Event [Member] | Restricted Stock Units [Member]
Number of Shares [Rollforward]
Granted			14,569
Awarded		(12,766)
Granted, Grant Date Fair Value			$ 1.1
Awarded, Grant Date Fair Value		$ 0.7

[1] Shares granted include accrued dividends.

[2] Shares granted include accrued dividends.