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, 2017	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Performance Shares [Member]
Number of Shares [Rollforward]
As of January 1			127,580	119,540	119,635	114,765
Granted	[1]			57,189	43,583	47,992
Awarded				0	0	(36,515)
Unearned Grant Award				(42,126)	(36,670)	0
Forfeited				(7,023)	(7,008)	(6,607)
As of December 31				127,580	119,540	119,635
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 52.56	$ 52.72	$ 48.26	$ 47.02
Granted	[1]			52.43	58.95	46.47
Awarded				0.00	0.00	42.01
Unearned Grant Award				52.70	45.41	0.00
Forfeited				53.45	53.49	48.29
As of December 31				$ 52.56	$ 52.72	$ 48.26
Restricted Stock Units [Member]
Number of Shares [Rollforward]
As of January 1			54,728	57,694	53,888	55,982
Granted	[2]			20,351	26,702	19,645
Awarded				(19,661)	(19,464)	(18,860)
Forfeited				(3,656)	(3,432)	(2,879)
As of December 31				54,728	57,694	53,888
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 51.79	$ 49.86	$ 44.47	$ 40.85
Granted	[2]			50.25	54.81	48.44
Awarded				44.33	41.44	37.64
Forfeited				52.87	51.52	45.92
As of December 31				$ 51.79	$ 49.86	$ 44.47
Subsequent Event [Member] | Performance Shares [Member]
Number of Shares [Rollforward]
Granted			41,755
Awarded		0
Performance Period (Years)		3 years	3 years
Granted, Grant Date Fair Value			$ 2.6
Subsequent Event [Member] | Restricted Stock Units [Member]
Number of Shares [Rollforward]
Granted			17,639
Awarded		(14,794)
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.