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Defined Contribution Plan, Maximum Cost Recognized Per Year

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Forfeiture rate

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Maximum number of days for leave of absence

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Number of continuous months worked

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Number of Offering Periods

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The maximum level of payout (150%) in shares upon the vesting of PSU's, as determined by ranking the average of the Company's three year performance with respect to three performance measures against the performance of companies included in a specific stock market index, excluding those with revenues of less than $1,000,000 or that are not in the health or insurance Global Industry Classification Standard codes.

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The minimum level of payout (0%) in shares upon the vesting of PSU's, as determined by ranking the average of the Company's three year performance with respect to three performance measures against the performance of companies included in a specific stock market index, excluding those with revenues of less than $1,000,000 or that are not in the health or insurance Global Industry Classification Standard codes.

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The target level of payout (100%) in shares upon the vesting of PSU's, as determined by ranking the average of the Company's three year performance with respect to three performance measures against the performance of companies included in a specific stock market index, excluding those with revenues of less than $1,000,000 or that are not in the health or insurance Global Industry Classification Standard codes.

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Share-based Compensation Arrangement by Share-based Payment Award, Number of Performance Measures

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Share-based Compensation Arrangement by Share-based Payment Award, Payout, Net-Pretax Earnings Connected To Acquisition Percentage

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Share-based Compensation Arrangement by Share-based Payment Award, Payout, Shareholder Return Percentage

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Share-based Compensation Arrangement by Share-based Payment Award, Performance Measurement Period

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Share-based Compensation Arrangement by Share-based Payment Award, Performance Period

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The highest quantity of shares an employee can purchase under the plan per period.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The weighted average of per share prices paid for shares purchased on the open market for issuance to employees under the plan.

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Number of share instruments newly issued under a share-based compensation plan.

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Percentage of vesting of share-based compensation awards.

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Purchase price of common stock expressed as a percentage of its fair value.

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Stock Based Compensation (Narrative) (Details)	1 Months Ended				12 Months Ended
Stock Based Compensation (Narrative) (Details)	Jan. 31, 2019 $ / shares shares	Jul. 31, 2018 USD ($) $ / shares shares	Jan. 31, 2018 $ / shares shares	Jul. 31, 2017 $ / shares shares	Dec. 31, 2018 USD ($) offering_period performance_metric $ / shares shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	May 31, 2017 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeiture rate					5.00%	5.00%	5.00%
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share) \| $ / shares					$ 93.20	$ 99.40	$ 80.24
Unrecognized compensation cost					$ 25,800,000
Unrecognized compensation cost expected to be recognized over a weighted-average period (in years)					1 year 1 month 28 days
Compensation expense					$ 36,000,000.0	$ 23,700,000	$ 22,300,000
Compensation expenses income tax benefit					$ 6,500,000	$ 8,300,000	$ 7,800,000
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payout level minimum					0.00%
Percentage of payout level maximum					200.00%
Weighted average grant date fair value (in dollars per share) \| $ / shares					$ 123.51	$ 112.23	$ 81.30
Unrecognized compensation cost					$ 22,100,000
Unrecognized compensation cost expected to be recognized over a weighted-average period (in years)					10 months 2 days
Compensation expense					$ 19,600,000	$ 10,500,000	$ 18,100,000
Compensation expenses income tax benefit					$ 3,100,000	$ 3,700,000	$ 6,300,000
Long-Term Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Company's common stock authorized to employees (in shares) \| shares								1,500,000
Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)					3 years
Long-Term Equity Incentive Plan \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payout level minimum					0.00%	0.00%	0.00%
Percentage of payout level maximum					200.00%	200.00%	200.00%
Performance period					3 years
Percentage of payout level target						100.00%	100.00%
Performance measures \| performance_metric					2
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Company's common stock authorized to employees (in shares) \| shares					5,000,000
Number of offering periods \| offering_period					2
Percentage of stock price purchased					90.00%
Participants' maximum contribution per offering period					$ 7,500
Participants' maximum contribution per year					$ 15,000.0
Number of continuous months worked					6 months
Non temporary employee requirement (months employed)					12 months
Maximum number of days for leave of absence					90 days
Maximum number of shares can be purchased each offering period per employee \| shares					5,000
Common shares issued \| shares		40,571	39,853	42,367
Discounted price of shares issued (in dollars per share) \| $ / shares		$ 89.31	$ 90.76	$ 84.71
Compensation expense					$ 1,500,000	$ 1,300,000	$ 1,300,000
Compensation expenses income tax benefit					$ 200,000	$ 200,000	$ 200,000
Subsequent Event \| Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares issued \| shares	42,950
Discounted price of shares issued (in dollars per share) \| $ / shares	$ 80.50
Tranche one \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
Tranche two \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
Tranche three \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
CEO and Executive Officers [Member] \| Long-Term Equity Incentive Plan \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period		30 months
Payout related to shareholder return percentage		60.00%
Payout related to net pre-tax earnings connected to acquisition percentage		40.00%
Allocated costs		$ 11,100,000
Director \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)					3 years
Director \| Tranche one \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
Director \| Tranche two \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
Director \| Tranche three \| Long-Term Equity Incentive Plan \| Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSU vesting terms for employees and directors					33.33%
Chief Executive Officer [Member] \| Long-Term Equity Incentive Plan \| Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated costs		$ 4,000,000.0