Share-based compensation plans - Grants in period and fair value assumptions for share-based compensation plans (Details)	6 Months Ended
Jun. 30, 2017 USD ($) $ / shares shares
Share options
Share-based compensation plans
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	10 years
2009 share award and incentive plan \| Restricted shares without performance criteria \| Director to Elect
Share-based compensation plans
Granted deferred shares (in shares) \| shares	116,540
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Restricted shares without performance criteria \| Vesting March 17, 2018
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Restricted shares without performance criteria \| Vesting March 17, 2019
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Restricted shares without performance criteria \| Vesting March 17, 2020
Share-based compensation plans
Granted deferred shares (in shares) \| shares	117,231
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Restricted shares without performance criteria \| Vesting March 17, 2021
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Restricted shares with performance criteria \| Vesting March 17, 2020
Share-based compensation plans
Granted deferred shares (in shares) \| shares	228,500
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
2009 share award and incentive plan \| Common Class A Vesting June 2018 \| Share options
Share-based compensation plans
Granted deferred shares (in shares) \| shares	40,900
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 13.45
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	29.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.50%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Payments \| $	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	2 years 6 months
2009 share award and incentive plan \| Common Class A Vesting June 2019 \| Share options
Share-based compensation plans
Granted deferred shares (in shares) \| shares	40,900
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 13.45
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	29.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.50%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Payments \| $	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	3 years 6 months
2009 share award and incentive plan \| Common Class A Vesting June 2020 \| Share options
Share-based compensation plans
Granted deferred shares (in shares) \| shares	40,900
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 13.45
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	30.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.77%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Payments \| $	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	4 years 6 months
2009 share award and incentive plan \| Common Class A Vesting June 2021 \| Share options
Share-based compensation plans
Granted deferred shares (in shares) \| shares	40,900
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 13.45
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	34.00%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.77%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Payments \| $	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	5 years 6 months

The per share price that the individual must pay to acquire shares under the deferred compensation arrangement.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualExercisePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendPayments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=bel_ShareAwardAndIncentivePlan2009Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=bel_RestrictedStockwithoutPerformanceCriteriaSharebasedPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=bel_DirectortoElectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=bel_VestingMarch182018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=bel_VestingMarch182019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=bel_VestingMarch182020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=bel_VestingMarch172021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=bel_DeferredCompensationWithPerformanceCriteriaShareBasedPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=bel_CommonClassAVestingJune2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=bel_CommonClassAVestingJune2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=bel_CommonClassAVestingJune2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=bel_CommonClassAVestingJune2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: