Shareholders' Equity - Company's Repurchasing Activity by Repurchase Plan (Detail) - $ / shares shares in Thousands	216 Months Ended	300 Months Ended
Dec. 31, 2018	Dec. 31, 2018
Non-Discretionary Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative shares repurchased		39,912
Average price per share		$ 35.20
Discretionary Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative shares repurchased	68,743
Average price per share	$ 44.66

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Section S99
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_StockRepurchasedDuringPeriodShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration