Benefit Plans - 401(k) Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Maximum
401(k) Match Plan
Employer match of employee contributions as percentage of employee's pay	5.00%	5.00%
Salaries and employee benefits
401(k) Match Plan
Matching employer contributions to 401(k) plan	$ 4.9	$ 4.6	$ 4.3

Amount of cost for defined contribution plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration