Consolidated Balance Sheets (Parenthetical) - £ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	£ 0.05	£ 0.05
Preferred stock, shares authorized, unlimited	Unlimited	Unlimited
Preferred stock, issued	328,184,640	328,184,640
Preferred stock, shares outstanding	328,184,640	328,184,640
Preferred stock, equivalent ordinary shares upon future consolidation outstanding	32,818,464	32,818,464
Common stock, par value	£ 0.50	£ 0.50
Common stock, shares authorized, unlimited	Unlimited	Unlimited
Common stock, issued	272,719,044	270,183,201
Common stock, outstanding	271,022,011	269,363,696
Treasury stock, shares	1,697,033	819,505