Debt - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2016	Dec. 31, 2015	Jun. 18, 2007
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt	$ 0		$ 900,000			$ (1,600,000)	$ 900,000
Line of Credit Facility, Description				Each advance would need to be repaid on the 30th day after the advance or, if earlier, on April 30, 2017, the expiration date of the Line of Credit.				Each advance would need to be repaid on the 30th day after the advance or, if earlier, on April 30, 2016, the expiration date of the Line of Credit.
Discretionary Line of Credit
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 100,000,000
Line of Credit Facility, Interest Rate Description				advances under the Line of Credit (if any) will bear interest at a variable rate equal to the higher of PNC's Prime Rate and the sum of the Federal Funds Open Rate plus 50 basis points.				advances under the Line of Credit (if any) will bear interest at a variable rate equal to the higher of PNC's Prime Rate and the sum of the Federal Funds Open Rate plus 50 basis points.
Line of Credit Facility, Amount Outstanding						0			$ 0
Debt Instrument, Basis Spread on Variable Rate				0.50%				0.50%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 500,000,000			$ 500,000,000		$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.45%			2.45%		2.45%
Carrying Value	$ 495,600,000		0	$ 495,600,000		$ 495,600,000	0		0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Repurchase of debt, face amount		19,800,000	18,400,000		$ 0
Debt Instrument, Interest Rate, Stated Percentage	6.70%			6.70%		6.70%				6.70%
Carrying Value	$ 593,900,000		612,600,000	$ 593,900,000		$ 593,900,000	612,600,000		612,800,000
Gains (losses) on extinguishment of debt		(1,600,000)					900,000
Other Assets [Member]
Debt Instrument [Line Items]
Ownership interest in the variable interest entities	$ 1,300,000		$ 1,300,000	$ 1,300,000		$ 1,300,000	1,300,000		$ 1,300,000
Net unrealized gains on forecasted transactions | 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)		$ 100,000					$ 200,000