Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Benefit Plans
Maximum contribution to defined contribution retirement plan by employee, as a percentage of compensation	75.00%
Employer's matching contribution as a percentage of employee's contribution	50.00%
Expense recognized	$ 8.7	$ 7.4	$ 6.3
Maximum
Benefit Plans
Maximum employer matching contribution to defined contribution retirement plan, as a percentage of compensation	6.00%

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The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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- Definition

Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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- Details

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