Consolidated Balance Sheets (Parenthetical) - £ / shares	Dec. 31, 2020	Dec. 31, 2019
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
Preferred stock, shares outstanding	0	289,317,460
Preferred stock, equivalent ordinary shares upon future consolidation outstanding		28,931,746
Common stock, par value	£ 0.50	£ 0.50
Common stock, shares authorized, unlimited	Unlimited	Unlimited
Common stock, issued	398,425,000	365,014,893
Common stock, outstanding	392,538,081	360,103,901
Treasury stock, shares	5,886,919	4,910,992