Dividend Restrictions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
New Hampshire's Statute
Restrictions on Dividends, Loans and Advances [Line Items]
Maximum percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared		$ 99.0	$ 75.0
Maximum dividends payable without prior approval of state regulators	151.9
Michigan's Statute
Restrictions on Dividends, Loans and Advances [Line Items]
Maximum percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared	70.0	69.0	70.0
Maximum dividends payable without prior approval of state regulators	$ 68.3
Additional dividend approval date	December 2013