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. 22, 2016	Jan. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Performance Shares [Member]
Number of Shares [Rollforward]
As of January 1			119,540	119,635	114,765	107,899
Granted	[1]			43,583	47,992	45,830
Awarded				0	(36,515)	(18,605)
Unearned Grant Award				(36,670)	0	(18,606)
Forfeited				(7,008)	(6,607)	(1,753)
As of December 31				119,540	119,635	114,765
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 52.72	$ 48.26	$ 47.02	$ 40.73
Granted	[1]			58.95	46.47	52.15
Awarded				0.00	42.01	35.10
Unearned Grant Award				45.41	0.00	35.10
Forfeited				53.49	48.29	47.26
As of December 31				$ 52.72	$ 48.26	$ 47.02
Restricted Stock Units [Member]
Number of Shares [Rollforward]
As of January 1			57,694	53,888	55,982	56,415
Granted	[2]			26,702	19,645	21,440
Awarded				(19,464)	(18,860)	(20,939)
Forfeited				(3,432)	(2,879)	(934)
As of December 31				57,694	53,888	55,982
Performance Period (Years)				3 years
Weighted-Average Grant Date Fair Value [Abstract]
As of January 1			$ 49.86	$ 44.47	$ 40.85	$ 36.61
Granted	[2]			54.81	48.44	43.41
Awarded				41.44	37.64	32.03
Forfeited				51.52	45.92	41.02
As of December 31				$ 49.86	$ 44.47	$ 40.85
Subsequent Event [Member] | Performance Shares [Member]
Number of Shares [Rollforward]
Granted			51,586
Awarded		0
Performance Period (Years)		3 years	3 years
Granted, Grant Date Fair Value			$ 2.7
Subsequent Event [Member] | Restricted Stock Units [Member]
Number of Shares [Rollforward]
Granted			17,396
Awarded		(17,608)
Granted, Grant Date Fair Value			$ 0.9
Awarded, Grant Date Fair Value		$ 0.8

[1] Shares granted include accrued dividends.

[2] Shares granted include accrued dividends.