Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands		12 Months Ended
	Jun. 01, 2017 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares) | shares		2,400,000
Common stock available for issuance (in shares) | shares		481,948
Stock-based compensation expense		$ 20,288	$ 11,948	$ 11,863
Weighted-average remaining contractual term, stock options outstanding		1 year 2 months 24 days	1 year 10 months 24 days
Weighted-average remaining contractual term, stock options vested and exercisable		1 year 2 months 24 days
Aggregate intrinsic value, stock options outstanding		$ 0
Aggregate intrinsic value, stock options vested and exercisable		10,800
Stock options vested (in shares) | shares			0
Intrinsic value, stock options exercised		2,500	$ 1,300	700
Cash received from stock option exercises		111	$ 491	$ 116
Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant-date intrinsic value		$ 31,500
Weighted-average grant-date fair value (in $ per share) | $ / shares		$ 108.32	$ 80.33	$ 134.37
Fair value of awards vested		$ 11,300	$ 9,300	$ 11,800
Share settlement under RSUs (in shares) | shares		32,716	37,488	50,952
Cash used for employee withholding tax payments		$ 3,500	$ 1,500	$ 5,100
Unamortized stock-based compensation expense		$ 29,300
Weighted average remaining amortization period		1 year 6 months 24 days	1 year 4 months 24 days
Restricted stock units (RSUs), performance-based [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares) | shares		87,458	33,244
Stock-based compensation expense		$ 7,300	$ 2,800
Unamortized stock-based compensation expense		7,600	$ 3,300
Performance Stock Units, Incentive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock options in years	5 years
Accelerated vesting period in years	4 years
Performance Share Units and Restricted Stock Units, Acquisition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 1,500
Restricted Stock Units and Performance Shares Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unamortized stock-based compensation expense					$ 8,600
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual life in years		10 years
Vesting period of stock options in years		3 years
Minimum | Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual life in years		1 year
Maximum | Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual life in years		3 years
RidgeWorth | Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares) | shares	65,561
RidgeWorth | Performance Stock Units, Incentive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares) | shares	35,148
Vesting Beginning With Year Two [Member] | Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of stock options in years	4 years
Common Stock | Restricted stock units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion ratio		1