Stockholders' Equity - Nonvested Restricted Stock Awards (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted stock awards
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares)	615,472	1,109,622	1,626,827
Awarded (in shares)	14,959	77,744	20,115
Vested (in shares)	(585,438)	(571,894)	(537,320)
Forfeited (in shares)	(13,891)
Outstanding, end of period (in shares)	31,102	615,472	1,109,622
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)	$ 58.71	$ 57.88	$ 57.66
Awarded (in dollars per share)	70.20	65.52	67.12
Vested (in dollars per share)	58.33	58.03	57.56
Forfeited (in dollars per share)	65.97
Outstanding, end of period (in dollars per share)	$ 68.00	$ 58.71	$ 57.88
Unrecognized compensation expense	$ 1.0
Unrecognized compensation expense, period of recognition	5 months 12 days
Restricted stock awards \| AFSC
Nonvested restricted stock awards and units
Awarded (in shares)		60,069
Restricted Stock Units (RSUs)
Nonvested restricted stock awards and units
Outstanding, beginning of period (in shares)	200,178	231,088	156,695
Awarded (in shares)	107,417	51,521	187,272
Vested (in shares)	(119,489)	(53,839)	(79,036)
Forfeited (in shares)	(24,817)	(28,592)	(33,843)
Outstanding, end of period (in shares)	163,289	200,178	231,088
Weighted average exercise price of nonvested restricted stock award and units
Outstanding, beginning of period (in dollars per share)	$ 61.65	$ 61.53	$ 54.88
Awarded (in dollars per share)	68.53	64.87	63.42
Vested (in dollars per share)	60.38	63.32	52.82
Forfeited (in dollars per share)	65.87	63.34	61.54
Outstanding, end of period (in dollars per share)	$ 66.46	$ 61.65	$ 61.53
Unrecognized compensation expense	$ 6.0
Unrecognized compensation expense, period of recognition	1 year 9 months 18 days

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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 number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made 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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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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-Publisher FASB
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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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-Publisher FASB
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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