Goodwill and Other Intangible Assets - Summary of gross carrying amounts - (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Other intangible assets
Gross carrying amount	$ 126,449	$ 100,274	$ 126,449
Accumulated amortization	(56,073)	(44,813)	(52,660)
Net carrying amount	70,376	55,461	$ 73,789
Amortization expense	3,413	$ 2,507
Estimated amortization expense for other intangibles for each of the next five quarters
June 30, 2018	3,401
September 30, 2018	3,400
December 31, 2018	3,400
March 31, 2019	3,156
June 30, 2019	3,144
Thereafter	$ 53,875
Minimum
Other intangible assets
Estimated useful lives	2 years
Maximum
Other intangible assets
Estimated useful lives	15 years