Condensed Consolidated Balance Sheets (Parenthetical) - £ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	£ 0.05	£ 0.05
Preferred stock, shares authorized, unlimited	Unlimited	Unlimited
Preferred stock, issued	289,317,460	289,317,460
Preferred stock, shares outstanding	289,317,460	289,317,460
Preferred stock, equivalent ordinary shares upon future consolidation outstanding	28,931,746	28,931,746
Common stock, par value	£ 0.50	£ 0.50
Common stock, shares authorized, unlimited	Unlimited	Unlimited
Common stock, issued	334,365,726	329,110,863
Common stock, outstanding	330,578,168	325,850,013
Treasury stock, shares	3,787,558	3,260,850

Preferred stock, equivalent ordinary shares upon future conversion outstanding.

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Face amount or stated value per share of common stock.

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Indicates that the number of common shares permitted to be issued by an entity's charter and bylaws is unlimited. The acceptable value is "Unlimited".

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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Indicates that the number of nonredeemable preferred shares, or preferred stock redeemable solely at the option of the issuer, permitted to be issued by an entity's charter and bylaws is unlimited. The acceptable value is "Unlimited".

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

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