Stockholders' Equity, Voting Rights, Stock Award and Incentive Plan (Details)	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2016 USD ($) VoteperShare Plan $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Stock Award and Incentive Plan [Abstract]
Number of incentive plans | Plan								2
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Actual stock based compensation expense			$ 1,037,000	$ 4,945,000	$ 1,644,000	$ 1,640,000	$ 1,639,000	$ 3,258,000	$ 4,923,000	$ 9,868,000
2016	$ 3,950,000							3,950,000
2017	2,492,000							2,492,000
2018	1,685,000							1,685,000
2019	1,348,000							1,348,000
2020	737,000							737,000
2021	497,000							497,000
2022	300,000							300,000
2023	127,000							127,000
2024	19,000							19,000
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
Compensation cost related to non-vested options not yet recognized	$ 7,900,000							$ 7,900,000
Stock options exercised (in shares) | shares	0				26,000			0	26,000
Proceeds from exercise of stock options					$ 1,167,000				$ 1,167,000
Tax benefit from exercise of stock options					$ 102,000				$ 102,000
Stock Options [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Term of nonqualified stock options								10 years
Restricted Stock Awards [Member]
Stock Award and Incentive Plan [Abstract]
RSA shares outstanding (in shares) | shares	427,290			553,100	688,550			427,290	688,550	553,100
Average weighted grant price (in dollars per share) | $ / shares	$ 65.72			$ 64.02	$ 67.34			$ 65.72	$ 67.34	$ 64.02
Restricted Stock Awards [Member] | Vesting in Three Years from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								30.00%
Award vesting period								3 years
Restricted Stock Awards [Member] | Vesting in Five Years from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								70.00%
Award vesting period								5 years
Restricted Stock Awards [Member] | Vesting in Year Four from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Five from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Six from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Seven from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Eight from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Nine from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Restricted Stock Awards [Member] | Vesting in Year Ten from Date of Grant [Member]
Stock Award and Incentive Plan [Abstract]
Award vesting percentage								10.00%
Q1 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2017	$ 685,000							$ 685,000
2018	464,000							464,000
2019	365,000							365,000
2020	219,000							219,000
2021	145,000							145,000
2022	93,000							93,000
2023	48,000							48,000
2024	7,000							7,000
Q2 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Actual stock based compensation expense		$ 1,036,000
2017	685,000							685,000
2018	453,000							453,000
2019	365,000							365,000
2020	204,000							204,000
2021	145,000							145,000
2022	93,000							93,000
2023	48,000							48,000
2024	7,000							7,000
Q3 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2016	1,186,000							1,186,000
2017	602,000							602,000
2018	402,000							402,000
2019	323,000							323,000
2020	169,000							169,000
2021	114,000							114,000
2022	66,000							66,000
2023	24,000							24,000
2024	5,000							5,000
Q4 [Member]
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
2016	691,000							691,000
2017	520,000							520,000
2018	366,000							366,000
2019	295,000							295,000
2020	145,000							145,000
2021	93,000							93,000
2022	48,000							48,000
2023	7,000							7,000
2024	$ 0							$ 0
Class A [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare								1
Class A [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares reserved for issuance under each plan (in shares) | shares	3,500,000							3,500,000
Class B [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare								10