Debt - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 26, 2015	Apr. 25, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2015	Jun. 18, 2007
Debt Instrument [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest			$ 368,500,000	$ 293,400,000	$ 664,100,000	$ 614,700,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			100,000,000		$ 100,000,000		$ 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 or 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		$ 0
Discretionary Line of Credit | Federal Funds Rate
Debt Instrument [Line Items]
Additional percentage on variable rate basis					0.50%		0.50%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt Instrument, Issuance Date	Jan. 26, 2015
Debt Instrument, Face Amount	$ 400,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	3.70%		3.70%		3.70%				6.70%
Proceeds from Debt, Net of Issuance Costs	$ 394,900,000
Additional Expenses Related to the Issuance	800,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	$ (12,900,000)
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		Apr. 25, 2014
Debt Instrument, Face Amount		$ 350,000,000
Debt Instrument, Interest Rate, Stated Percentage		4.35%	4.35%		4.35%
Proceeds from Debt, Net of Issuance Costs		$ 346,300,000
Additional Expenses Related to the Issuance		700,000
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)		$ (1,600,000)
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			6.70%		6.70%
Repurchase of debt, face amount					$ 0	0	$ 44,300,000
(Gains) losses on extinguishment of debt					0	0	4,800,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 | Net unrealized gains on forecasted transactions
Debt Instrument [Line Items]
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)							$ 500,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					0	$ 0
Surplus Note [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Minimum surplus requirement under debt covenant			$ 50,000,000
Debt principal expected to be paid								$ 2,200,000
London Interbank Offered Rate (LIBOR) [Member] | Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Additional percentage on variable rate basis			2.25%
Other Assets [Member]
Debt Instrument [Line Items]
Ownership interest in the variable interest entities			$ 1,300,000		$ 1,300,000
Scenario, Forecast [Member] | Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Debt principal expected to be paid								$ 25,000,000