Outstanding Debt Consisted (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
7.75% Subordinated Notes due 2045
Debt Instrument [Line Items]
Subordinated Notes percentage	7.75%	7.75%
Subordinated Notes due date	2045	2045
7.875% Subordinated Notes due 2047
Debt Instrument [Line Items]
Subordinated Notes percentage	7.875%
Subordinated Notes due date	2047