ACQUISITIONS - Capstone Investment Management, LLC (Details) - USD ($) $ in Thousands			12 Months Ended
	May 01, 2017	Feb. 17, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ACQUISITIONS
Cash paid for acquisitions, net			$ (2,845)	$ 11,289	$ (57,083)
Goodwill			$ 137,090	$ 101,315	$ 75,953
Other customer relationship intangibles
ACQUISITIONS
Intangible asset amortization period			10 years
Capstone
ACQUISITIONS
Cash paid for acquisitions, net	$ 756
Purchase price	1,750
Intangible Assets, Net (Including Goodwill)	610
Loan assumed	$ 644
Period over which remaining balance of loan assumed will be earned	2 years
Capstone | Other customer relationship intangibles
ACQUISITIONS
Intangible asset amortization period	15 years
Intangible Assets, Net (Including Goodwill)	$ 1,140
Capstone | Customer relationship and goodwill
ACQUISITIONS
Intangible asset amortization period	15 years
First Service Capital Management, Inc.
ACQUISITIONS
Cash paid for acquisitions, net		$ 1,400
Purchase price		1,750
Goodwill		$ 660
Period for tax deduction purposes for goodwill and intangible assets		15 years
First Service Capital Management, Inc. | Other customer relationship intangibles
ACQUISITIONS
Intangible asset recorded		$ 1,090
Intangible asset amortization period		15 years