Employee Benefits (Details Textual) $ / shares in Units, CAD in Millions		1 Months Ended	12 Months Ended
	Apr. 26, 2017 shares	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 CAD shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2017 CAD shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amount of shares purchased in the open market			$ 0		$ 5,634,000	$ 0
Dividends payable			7,357,000		6,679,000
Accrued dividends			1,717,000		1,767,000
Accrued liability			1,800,000		$ 1,700,000
Vesting period					6 years
Litigation Settlement, Expense			$ 4,000,000	CAD 5.0	$ 0	0
Deferred Compensation Share Based Payments | Share-based Compensation Award, Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of deferred share units vesting			20.00%	20.00%
Deferred Compensation Share Based Payments | Share-based Compensation Award, Tranche Two
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of deferred share units vesting			40.00%	40.00%
Deferred Compensation Share Based Payments | Share-based Compensation Award, Tranche Three
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of deferred share units vesting			60.00%	60.00%
Deferred Compensation Share Based Payments | Share Based Compensation Award Tranche Four
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of deferred share units vesting			80.00%	80.00%
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total number of shares that may be issued under the stock based compensation Plan (including predecessor plans to the Plan) | shares			4,648,100				4,648,100
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized | shares	250,000
Shares remain available for issuance | shares			454,000				454,000
Vested, shares | shares			187,295	187,295
Remaining unrecognized compensation cost			$ 22,100,000
Remaining unrecognized compensation cost recognized over a remaining weighted average period			1 year 11 months	1 year 11 months
Number of vested shares from employees on the date vesting | shares			87,946	87,946
Nonvested restricted shares (in shares) | shares			519,375		607,501		519,375
Compensation expense recorded					$ 15,954,000	17,574,000
Restricted Stock | Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted to employees vesting period			4 years	4 years
Restricted Stock | Director
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted to employees vesting period			1 year	1 year
Canadian Plan | Westwood International Advisors Inc
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares remain available for issuance | shares			52,000				52,000
Fund purchases of common stock			$ 8,000,000				CAD 10.0
Purchases of common stock with respect to awards granted			$ 3,400,000
Share price | $ / shares			$ 66.21
Number of shares purchased in the open market | shares			23,822	23,822
Vested, shares | shares			22,091	22,091
Fair value of vested shares			$ 1,200,000
Treasury Stock, Common, Shares | shares			33,327				33,327
Purchase of treasury stock, shares | shares			1,325,625	1,325,625
Remaining unrecognized compensation cost			$ 676,000
Remaining unrecognized compensation cost recognized over a remaining weighted average period			1 year 9 months	1 year 9 months
Compensation expense recorded			$ 232,000		$ 524,000	145,000
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Adjusted pre-tax income		$ 24,000,000.0
Share Pretax Income Goal Sliding Scale Award			$ 34,000,000
Performance​ Based​ Restricted​ Shares​ For Future Performance Years
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Nonvested restricted shares (in shares) | shares			16,300				16,300
Deferred Compensation Share Based Payments
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted shares granted to employees vesting period			6 years	6 years
Accrued liability			$ 632,000
Deferred share units, issued and outstanding | shares			10,796	10,796
Minimum [Member] | Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Adjusted pre-tax income			$ 24,000,000
Share Pretax Income Goal Sliding Scale Award			$ 30,300,000
Percentage of Pretax income Goal Earned			25.00%	25.00%
Maximum [Member] | Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Pretax Income Goal Sliding Scale Award			$ 42,500,000
Percentage of Pretax income Goal Earned			0.00%	0.00%
Mutual Fund [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Mutual fund vesting period			2 years	2 years	1 year
Service period of mutual fund share incentive award			3 years	3 years
Compensation expense recorded			$ 1,200,000		$ 1,300,000	$ 1,200,000
Mutual Fund [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Service period of mutual fund share incentive award			2 years	2 years
Benefit Plans [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			4 years	4 years
Percentage of compensation			6.00%	6.00%